From 'Shelley at Oxford' (PDF 13.1mb) by Thomas Jefferson Hogg, 1832/33, (1904 reprint from the original magazine articles, pp110-115):

[at Oxford 1810/11] His food was plain and simple as that of a hermit, with a certain anticipation, even at this time, of a vegetable diet, respecting which he afterwards became an enthusiast in theory, and in practice an irregular votary. . . .his mode of living already offered a foretaste of the studious seclusion and absolute renunciation of every luxurious indulgence which ennobled him a few years later.
[Hogg was at Oxford with Shelley and this account was well respected by those who knew him (unlike Hogg's much later, and much discredited 'Life of Shelley') - he indicates that at Oxford Shelley was already concerned about the practice of eating animals. There are also references to an interest in Pythagoreans, transmigration of souls etc. Another early friend suggested that Shelley was already interested in ideas about metempsychosis 'long before' he went up to Oxford.]

From: Shelley - a life story, by Edmund Blunden, 1946, p74:

. . . but Shelley had another choice. The ideas of William Godwin had already influenced his own; and [the poet Robert] Southey, presumably once a student of Godwin, mentioned that the philosopher was still living. On January 3rd, 1812, as one who has been put in touch with the miraculous, Shelley turned from many difficulties to the composition of one of his queer "first letters." [Shelley was in Keswick in the English Lake District at this time, moving to Dublin in February. William Godwin was in close contact with at least two vegetarian authors in London, and himself adopted the vegetable diet for some periods during the very early 19th century. He may well have encouraged Shelley's further reading on this, but we only have Shelley's letters to Godwin (PDF 4mb), not Godwin's to Shelley.]

From The Heretics Feast, (later re-published as 'A History of Vegetarianism') by Colin Spencer, 1993, p.247:

. . . [March 12, 1812, in Grafton Street, Dublin] his wife [Harriet writing to Miss Nugent] wrote 'we have forsworn meat and adopted the Pythagorean system. About a fortnight has elapsed since the change - we are delighted with it'. . . On 15 March Harriet writes a note to an Irish friend, Mrs. Nugent, inviting her to dinner: 'expects the pleasure of her company to dinner, 5 o'clock, as a murdered chicken has been prepared for her repast'.

From Letters from Percy Bysshe Shelley (& some by Harriet) to Elizabeth Hitchener - Vol.2, 1812 (full text PDF 5.5mb - privately printed 1890)

17 Grafton Street, Dublin. [Saturday], March 14, 1812. [Written by Harriet]

Our living is different to those worldlings, and you may or not adopt it as you think fit. You do not know that we have forsworn meat, and adopted the Pythagorean system. About a fortnight has elapsed since the change, and we do not find ourselves any the worse for it. What do you think of it ? Many say it is a very bad plan: but, as facts go before arguments, we shall see whether the general opinion is true or false. We are delighted with it, and think it the best thing in the world. As yet there is but little change of vegetables; but the time of year is coming on when there will be no deficiency.

From Shelley and the Revolution in Taste, Timothy Morton, 1994, p.63 (this appears to have been written to Miss Nugent):

Writing from Nantgwillt, in Radnorshire, Wales, one month later, on 16 April, Harriet recalls the journey back to Holyhead: "We did not eat anything for 36 hours all the time we were on board, and immediately began upon meat; you will think this very extraordinary, but Percy and my sister suffered so very much by the voyage."

From: Shelley - a life story, by Edmund Blunden, 1946:

. . . October and November, 1812, were interesting months to Shelley because Harriet and he saw much of William Godwin at home [in London]. Harriet as usual was ready to love all the new acquaintances. . . . "There is another daughter . . . who is now in Scotland" Harriet noted . . . [this was Mary who would eventually replace Harriet...] . . . Through William Godwin Junior, on Guy Fawkes night, Shelley met a man of originality, perhaps the king of the vegetarians; the child took him to see the fireworks arranged by John Frank Newton, author of "The Return to Nature" recommending vegetable diet. In fact the master vegetarian was Dr. William Lambe, to whom he intended to write three more books on the theme.

The Shelleys spent the winter of 1812/13 in North Wales and Ireland. In April 1813, they were back in London. Shelley's first long poem, Queen Mab was published [full text], we do not have a precise date for that but it appears to have been early summer. Queen Mab itself contains several vegetarian references:

. . . . . . . . . . . . . . . . . No longer now
He slays the lamb that looks him in the face,
And horribly devours his mangled flesh;
Which, still avenging nature's broken law,
Kindled all putrid humours in his frame,
All evil passions, and all vain belief,
Hatred, despair, and loathing in his mind,
The germs of misery, death, disease, and crime.
No longer now the winged habitants,
That in the woods their sweet lives sing away,
Flee from the form of man; but gather round,
And prune their sunny feathers on the hands
Which little children stretch in friendly sport
Towards these dreadless partners of their play.

From: Shelley - a life story, by Edmund Blunden, 1946, p.99:

One of the notes [to Queen Mab] extends to the length of an essay and was separately published with the title "A Vindication of Natural Diet." [full text] Much like Milton in "Paradise Lost," Shelley connected the original happiness of man and the way to regain it with vegetarianism. He had read much on the question, and in several languages; he thought that he might put the matter on a scientific footing. "Man resembles no carnivorous animal. There is no exception, unless man be one, to the rule of herbivorous animals having cellulated colons." It was all new and exciting. Shelley employed his friend J. F. Newton as an instance of the blessings to be derived from the faith; did anybody fear that parenthood would be weakened by the vegetable diet? then look at Newton. His "children are the most beautiful and healthy creatures it is possible to conceive; the girls are perfect models for a sculptor; their dispositions are also the most gentle and conciliating; the judicious treatment which they experience in other points may be a correlative cause of this." The Newtons in fact were among those who are now pleasantly caricatured by James Thurber, but Shelley was not a practical nudist. We see him, in passing, at his own table, and perhaps wishing for the kitchen garden of his father: "The pleasure or taste to be derived from a dinner of potatoes, beans, peas, turnips, lettuces, with a dessert of apples, gooseberries, strawberries, currants, raspberries, and in winter, oranges, apples, and pears, is far greater than is supposed."

In The Bloodless Revolution, 2006, p 372, Tristram Stuart quotes the rather more sensationalist account of J. F. Newton's house from Hogg's (much discredited) 1858 biography of Shelley - though he fails to mention either the date (spring-1813) or the location (Grosvenor Square, London):

When Percy Bysshe Shelley (1792-1822) first brought his university friend Thomas Jefferson Hogg to the home of his vegetarian companions, Shelley thrust him forward as the door opened. Hogg, who ended up writing Shelley's biography, described what he saw as 'a strange spectacle': there were 'five naked figures in the passage advancing rapidly to meet us'. 'As soon as they saw me,' he wrote, 'they uttered a piecing cry, turned round and ran wildly upstairs, screaming aloud.' These nudist enthusiasts were the children of John Frank Newton, author of The Return to Nature (1811), who was trying to return his whole family to nature by practising a combination of 'nakedism' and vegetarianism. ' The custom of flesh eating as much as that of covering our persons with clothes,' preached Newton, was an accidental feature of human development which 'appears to have arisen from the migration of man to northern climes'.
[full account: The life of Percy Bysshe Shelley (PDF 20mb) by T. J. Hogg. See p.287 for the above scene - Hogg continued this account with: 'I was presented to a truly elegant family, and I found everything in the best taste, and was highly gratified with my reception.' Despite the impression given in the extract above, Hogg devotes several pages to his admiration for the family.]

. . . Newton's vegetarian ally Dr. William Lambe (1765-1847) opined that . . . these children are unparalleled. Lambe's membership of the Royal Society of Physicians and Fellowship of St. John's College, Cambridge lent such claims scientific clout.

from: The life of Percy Bysshe Shelley (PDF 20mb) by T. J. Hogg. pp.414-420:

Shelley fed much on pulse at different periods, . . . through a calm, deliberate choice, and a sincere conviction of the propriety and superior salubrity of such food. His letters inform us, that he had occasionally restricted himself in great measure, if not entirely, to a vegetable diet. What first suggested to him the abstinence from flesh does not anywhere appear ; whether his own feelings and reflections, or the advice of others given orally, or in books. It was not until the spring of the year 1813 [sic -as above it was 1812] that he entered upon a full and exact course of vegetable diet. His Pythagorean, or Brahminical, existence, and his intimate association with the amiable and accomplished votaries of a Return to Nature [ie the Newton family as above, and J. F. Newton's book 'Return to Nature'], was perhaps the prettiest and most pleasing portion of his poetical, philosophical, and lovely life. His nutriment had ever been, and always was, simple ; consisting, as has been already mentioned, principally of bread eaten by itself, or with some very slight and frugal condiment. Spirituous liquors he never tasted ; beer, rarely. He never called for, purchased, or drew, wine for his own drinking ; but if it came in his way, and the company was not disagreeable to him, he would sit at table a while after dinner, and take two or three glasses of any white wine, uniformly selecting the weakest.

[ . . . ]

For some months, for some years [an exaggeration], I was in the thick of it, for I lived much with a select and most estimable society of persons, who had returned to nature, . . . I conformed ; not through faith, but for good fellowship, and because it was an agreeable . . . change. . . . .Certainly their vegetable dinners were delightful ; elegant and excellent repasts ; . . Flesh, fowl, fish, game, never appeared ; nor eggs bodily in their individual capacity, nor butter in the gross : the two latter articles were admitted into cookery, it is true, but as sparingly as possible, and their presence was provisional, interlocutory, under protest, as culinary aids not approved of, and soon to be dispensed with. The injunction extended to shell-fish. . . . We had soups in great variety, that seemed the more delicate from the absence of meat. There were vegetables of every kind, the finest and best of the kind, dressed with care and skill ; either plainly or stewed, and otherwise artfully and scientifically arranged and disguised. Puddings, tarts, confections, sweets, abounded. Cheese was under the ban, — anathematized, excommunicate. Milk and cream might not be taken unreservedly; however, they were allowed to form ingredients in puddings, and to be poured sparingly into tea, as an indulgence to the weakness of neophytes, tender plants. Fruits of every description were welcomed,—hailed rapturously, received with plaudits, as if the goddess. Nature, herself stood bodily before her votaries. We luxuriated, ran riot in tea and coffee, and sought variety occasionally in cocoa and chocolate. Bread and butter and buttered toast were eschewed ; but bread-cakes, plain seed-cakes, were liberally divided amongst the faithful.

From: Shelley - a life story, by Edmund Blunden, 1946, p.103-6:

At the end of June [1813] Harriet managed to have her first baby without disaster, and within a month the little family were living "merely for convenience" at High Elms House, Bracknell. Like the girl whose spirit was schooled by Queen Mab, the baby was called (Eliza) Ianthe . . . [Blunden here quotes extracts from Peacock below] It was the coterie of which a glimpse was given in the notes to "Queen Mab," centred in the London house of J. F. Newton - in Chester Street, Grosvenor Square.
A sister of Mrs. Newton ... made a greater impression on him still [more accurately it was the sister's daughter who interested Shelley...]. Harriet de Boinville . . .[her son married the] daughter of the vegetarian Dr. Lambe [and she owned the house at Bracknell]. At Bracknell they entertained a number of revolutionaries and enjoyed their discussions, which Shelley shared.

from ' Memoir of Shelley' (PDF 13mb) by Thomas Love Peacock, originally published 1858, extracts from 1909 edition, pp.28-30

. . . he was residing at Bracknell, and invited me to visit him there. This I did, and found him with his wife Harriet, her sister Eliza, and his newly-born daughter lanthe. . . . At Bracknell, Shelley was surrounded by a numerous society, all in a great measure of his own opinions in relation to religion and politics, and the larger portion of them in relation to vegetable diet. But they wore their rue with a difference. Every one of them adopting some of the articles of the faith of their general church, had each nevertheless some predominant crotchet of his or her own, which left a number of open questions for earnest and not always temperate discussion. I was sometimes irreverent enough to laugh at the fervour with which opinions utterly unconducive to any practical result were battled for as matters of the highest importance to the well-being of mankind ; Harriet Shelley was always ready to laugh with me, and we thereby both lost caste with some of the more hot-headed of the party. [Peacock then refers at some length to John Frank Newton who was a frequent visitor to Bracknell. Later - pp.38/39 - Peacock takes the usual and predictable meat-eater's view that Shelley's diet caused him both physical and mental health problems - though it must be acknowledged that Shelley's diet at this particular time does not seem to have been a very healthy balance. We have learned a lot about nutrition in the last 200 years.....]

Note: some writers have confused the commentaries by Hogg and Peacock. Hogg states clearly that he never went to Bracknell, so he was writing about the Newtons in London. Peacock wrote about Bracknell. The two families were related and no doubt visited each other often.
See: Family Trees of the Bracknell/London/Windsor group

By the end of the summer of 1813 the Shelleys had moved back to London, in Pimlico, then again to the Lake District. Shelley returned to Bracknell for a month, alone, early 1814. In July 1814 Shelley left Harriet and eloped with Mary Wolstoncraft Godwin

from ' Memoir of Shelley' (PDF 13mb) by Thomas Love Peacock, originally published 1858, extracts from 1909 edition, pp.54-55

[during a boating trip up the Thames, late-August 1815, Shelley feeling unwell] . . .I told him, ' If he would allow me
to prescribe for him, I would set him to rights." He asked, ' What would be your prescription ? ' I said, ' Three mutton chops, well peppered' He said, 'Do you really think so?' I said, ' I am sure of it." He took the prescription ; the success was obvious and immediate., [in a letter to Hogg soon afterwards, September 1815, Shelley put his improved health down to the fresh air during the river trip, making no mention of Peacock's claimed dietary improvement, though there is no reason to doubt that there was another temporary diversion.]

from Mary Shelley - Romance and Reality, by Emily W. Sunstein, 1898, p.104:

[referring to Bishopsgate, August 1815 to May 1816] . . Shelley ... ate when he felt like it, perhaps standing with a book in one hand. "Mary, have I dined?" he would sometime ask. Like protesters against impure societies throughout the ages, he abstained from meat and alcohol; she laid in a store of vegetarian foods, occasionally made him a passable pudding, without sugar, which they boycotted because it came from slave plantations. She liked her tea sandwiches cut neatly, but dinner with proper courses was a rarity unless they had company, and throughout their union friends complained about the quality of her table.

Shelley's essay 'On the Vegetable System of Diet' is believed to have been written in 1815, but not published until the 1920s - an extract:

It is evident that those who are necessitated by their profession to trifle with the sacredness of life, and think lightly of the agonies of living beings, are unfit for the benevolence and justice which is required for the performance of the offices of civilised society. They are by necessity brutal, coarse, turbulent and sanguinary. Their habits form an admirable apprenticeship to the more wasting wickedness of war, in which men are hired to mangle and murder their fellow beings by thousands, that tyrants and countries may profit. How can he be expected to preserve a vivid sensibility to the benevolent sympathies of our nature, who is familiar with carnage, agony and groans? The very sight of animals in the fields who are destined to the axe must encourage obduracy if it fails to awaken compassion. The butchering of harmless animals cannot fail to produce much of that spirit of insane and hideous exultation in which news of a victory is related altho' purchased by the massacre of a hundred thousand men.

If the use of animal food be, in consequence, subversive to the peace of human society, how unwarrantable is the injustice and barbarity which is exercised toward these miserable victims. They are called into existence by human artifice that they may drag out a short and miserable existence of slavery and disease, that their bodies may be mutilated, their social feelings outraged. It were much better that a sentient being should never have existed, than that it should have existed only to endure unmitigated misery. (The attachment of animals to their young is very strong. The monstrous sophism that beasts are pure unfeeling machines, and do not reason, scarcely requires a confutation.)

from Alastor or The Spirit of Solitude (early 1816):

If no bright bird, insect, or gentle beast
I consciously have injured, but still loved
And cherished these my kindred - then forgive
This boast, beloved brethren, and withdraw
No portion of your wonted favour now

Mary Shelley started the story of Frankenstein in Switzerland during the summer of 1816, and finished the novel in England in May 1817, and it was published in January 1818.

from The Sexual Politics of Meat by Carol J. Adams, 1990 (quote from 2000 edition)p.120:

chapter 6: Frankenstein's Vegetarian Monster
Frankenstein's monster was a vegetarian. . . . For a work that has received an unusual amount of critical attention over the past thirty years, in which almost every aspect of the novel has been scrutinized, it is remarkable that the Creature's vegetarianism has remained outside the sphere of commentary.

The Creature's vegetarianism not only confirms its inherent, original benevolence, but conveys Mary Shelley's precise rendering of themes articulated by a group of her contemporaries whom I call "Romantic vegetarians." ... The myths of Adam and Eve and Prometheus, clearly evoked in the novel, were interpreted in a vegetarian framework during the romantic period as being about the introduction of meat eating.

... In a ringing emotional speech the Creature enunciates its dietary principles . . . "My food is not that of man; I do not destroy the lamb and the kid, to glut my appetite; acorns and berries afford me sufficient nourishment. . . ."

Extract from the introduction to 'Mary Shelley Frankenstein 1818 text' - edited and introduction by Prof. Marylin Butler, Oxford University Press, 1993:

". . . After Frankenstein vows to hunt down his 'progeny', the Creature nurtures Frankenstein to keep him alive, feeding him for example with a dead hare; only when killing for Frankenstein does the vegetarian Creature kill for food."

From: Shelley - a life story, by Edmund Blunden, 1946, p.154:

In December 1816, the tumultuous historical painter Benjamin Haydon . . . was invited to dine and to meet Shelley. Haydon arrived late and found his place opposite "a hectic, spare, weakly, yet intellectual-looking creature, carving a bit of broccoli or cabbage on his plate as if it had been the substantial wing of a chicken." It was a pity that Haydon had not arrived later still. He was at this period furiously clinging to orthodox Christianity; he heard the vegetarian opposite "saying in the most feminine and gentle voice, 'As to that detestable religion, the Christian....' ".

[Oct/Nov 1817] Mr. Baxter to his surprise found Shelley's personal life so frugal, sound and fine. . .

from Percy Bysshe Shelley: A Monograph, by Henry S. Salt (1888), p.243:

. . . at Marlow in 1817, we find Shelley still persevering in the reformed diet, though not without occasional lapses, if we are to believe his biographers Hogg and Peacock. The former gives a humorous account of an occasion when, in the dearth of other food, Shelley was induced to try fried bacon, and found it very good ; and Peacock asserts that during a boating excursion, in 1815, his prescription of "three mutton chops, well peppered," was of great service to Shelley's health. Nevertheless, Leigh Hunt reports him in 1817, when living at Marlow, as " coming home to a dinner of vegetables, for he took neither meat nor wine."

from The Revolt of Islam (written at Marlow, 1817):

Never again may blood of bird or beast
Stain with its venomous stream a human feast,
To the pure skies in accusation steaming.

From: Shelley - a life story, by Edmund Blunden, 1946:

[August 1818 breakfast in Florence] figs - very fine - and peaches

[Nov/Dec 1818 in Pompeii] We see the group of Shelleyans sitting under the portico of the temple of Jupiter, lunching on oranges, figs, medlars and bread, and enjoying a scene . . .

from Prometheus Unbound (written in Italy 1818/19):

I wish no living thing to suffer pain.

from Percy Bysshe Shelley: A Monograph, by Henry S. Salt (1888), p.243:

the poetic Letter to Maria Gisborne,' written in 1820— "Though we eat little flesh, and drink no wine."

Shelley drowned in July 1822, one month before his 30th birthday.

The last few years:

In 1817 Shelley was trying to obtain custody of his two children by his first wife Harriet, who had died in December 1816. This was rejected by the Lord Chancellor, Lord Eldon, because of Shelley's 'immorality' in living with Mary Godwin for the previous two years (they had since married). The prospect of the children being raised vegetarian would not have helped with the highly reactionary Lord Eldon, and Mary also became concerned that her own two children could be taken away from them.

In 1818 the Shelleys fled, permanently, to Italy where they were socially isolated, and Mary in particular recorded her loneliness. After a year or two they did make a new circle of friends, but there is no record of them ever meeting any other vegetarians in Italy.

By 1820 Shelley was becoming concerned about his health and was advised by a (non-vegetarian) doctor that a) - there was nothing seriously wrong with him, and b) - that he should change his lifestyle. Shelley was easily clever enough to see that b did not follow a, but finally seems to have given in to the all-round pressure for his last two years.

from Shelley's Vegetarianism (PDF 1.2mb) by W.E.A. Axon, 1890, pp.7-8:

The following letter from the late Sir Percy Shelley [Shelley's son, aged 2 when he died] may be cited :—
Boscombe Manor, Bournemouth, Hants, ... Nov. 14, 1883.
Dear Mr. Kegan Paul,

. . . I think I remember my mother telling me that he [Shelley] gave it up to a great extent in his later years - not from want of faith, but from the inconvenience. I made two attempts when I was young myself - each time I was a strict vegetarian for three months - but it made me very fat and I gave it up. That was my only reason, and it took me several days to overcome my disgust for animal food when I returned to it.
. . . . . . . —Yours, very sincerely, Percy F. Shelley.