Shelley's unconventional life and uncompromising idealism, combined with his strong disapproving voice, made him an authoritative and much-denigrated figure during his life and afterward. He became an idol of the next two or three or even four generations of poets, including the important Victorian and Pre-Raphaelite poets Robert Browning, Alfred, Lord Tennyson, Dante Gabriel Rossetti, Algernon Charles Swinburne, as well as Lord Byron, Henry David Thoreau, W. B. Yeats, Aleister Crowley and Edna St. Vincent Millay, and poets in other languages such as Jan Kasprowicz, Rabindranath Tagore, Jibanananda Das and Subramanya Bharathy.
Henry David Thoreau's civil disobedience and Mohandas Karamchand Gandhi's passive resistance were influenced and inspired by Shelley's nonviolence in protest and political action. It is known that Gandhi would often quote Shelley's Masque of Anarchy, which has been called "perhaps the first modern statement of the principle of nonviolent resistance."
Shelley wrote several essays on the subject of vegetarianism, the most prominent of which were "A Vindication of Natural Diet" (1813) and "On the Vegetable System of Diet".
Shelley, in heartfelt dedication to sentient beings, wrote: "If the use of animal food be, in consequence, subversive to the peace of human society, how unwarrantable is the injustice and the 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"; "Never again may blood of bird or beast/ Stain with its venomous stream a human feast,/ To the pure skies in accusation steaming"; and "It is only by softening and disguising dead flesh by culinary preparation that it is rendered susceptible of mastication or digestion, and that the sight of its bloody juices and raw horror does not excite intolerable loathing and disgust." In Queen Mab: A Philosophical Poem (1813) he wrote about the change to a vegetarian diet: "And man ... no longer now/ He slays the lamb that looks him in the face,/ And horribly devours his mangled flesh."
Shelley was a strong advocate for social justice for the "lower classes". He witnessed many of the same mistreatments occurring in the domestication and slaughtering of animals, and he became a fighter for the rights of all living creatures that he saw being treated unjustly.