Ruskin was born in London, and raised in south London. He was educated at the University of Oxford (Christ Church), where he was awarded a prize for poetry, his earliest interest. In 1848, Ruskin married Effie Gray. Their marriage was annulled in 1854 on grounds of non-consummation. (She later married the artist John Everett Millais). Ruskin later fell deeply in love with Rose la Touche. He met her in 1858, when she was only nine years old, proposed to her eight years later, and was finally rejected in 1872.

Ruskin taught at the Working Men's College in London and was the first Slade Professor of Fine Art at Oxford, from 1869 to 1879. There he became friendly with Lewis Carroll, another don, and was photographed by him. After the parting of Carroll and Alice Liddell, she and her sisters pursued a similar relationship with John Ruskin, as detailed in Ruskin's Praeterita. Ruskin College, Oxford is named after him.

Upon the death of his father (who was a wealthy wine merchant), Ruskin declared that it was not possible to be a rich socialist and gave away most of his inheritance. He was friends with Sir Henry Acland. He founded the charity known as the Guild of St George in the 1870s and endowed it with large sums of money as well as a remarkable collection of art. He also gave the money to enable Octavia Hill to begin her practical campaign of housing reform.

Ruskin spent much of his later life at a house called Brantwood, on the shores of Coniston Water located in the Lake District of England.

While still a student at Oxford, Ruskin met Joseph Mallord William Turner, whom he began to defend against critics in an 1836 essay. His Modern Painters series was responsible for the early popularity of the artist as well as the pre-Raphaelite movement.

He also worked with the artists Rossetti, Millais, Holman Hunt, John Brett, Burne-Jones and John William Inchbold.

In 1878 he wrote a review of a painting by James Whistler in which he accused the painter of "throwing a pot of paint in the face of the public" that led to a famous libel case. Ruskin lost, though the award of damages was only one farthing, and his reputation was tarnished which may have accelerated his mental decline. He suffered from a number of mental breakdowns as well as delirious visions.

His later works influenced many Trade Union leaders of the Victorian era. He was also the inspiration for the Arts and Crafts Movement, the founding of the National Trust, the National Art Collections Fund and the Society for the Protection of Ancient Buildings.

He wrote over 250 works, which tended to connect art history to topics ranging from science, literary criticism, environmental conditions, and mythology. He is well known for his essay on economy Unto This Last, the essay The Nature of Gothic, and the early fantasy novel The King of the Golden River.

Ruskin's views on art, wrote Kenneth Clark, "cannot be made to form a logical system, and perhaps owe to this fact a part of their value." Certain principles, however, remain consistent throughout his work and have been summarized in Clark's own words as the following:

1. That art is not a matter of taste, but involves the whole man. Whether in making or perceiving a work of art, we bring to bear on it feeling, intellect, morals, knowledge, memory, and every other human capacity, all focused in a flash on a single point. Aesthetic man is a concept as false and dehumanizing as economic man.
2. That even the most superior mind and the most powerful imagination must found itself on facts, which must be recognized for what they are. The imagination will often reshape them in a way which the prosaic mind cannot understand; but this recreation will be based on facts, not on formulas or illusions.
3. That these facts must be perceived by the senses, or felt; not learnt.
4. That the greatest artists and schools of art have believed it their duty to impart vital truths, not only about the facts of vision, but about religion and the conduct of life.
5. That beauty of form is revealed in organisms which have developed perfectly according to their laws of growth, and so give, in his own words, 'the appearance of felicitous fulfilment of function.'
6. That this fulfilment of function depends on all parts of an organism cohering and cooperating. This was what he called the 'Law of Help,' one of Ruskin's fundamental beliefs, extending from nature and art to society.
7. That good art is done with enjoyment. The artist must feel that, within certain reasonable limits, he is free, that he is wanted by society, and that the ideas he is asked to express are true and important.
8. That great art is the expression of epochs where people are united by a common faith and a common purpose, accept their laws, believe in their leaders, and take a serious view of human destiny."
(Kenneth Clark, "A Note on Ruskin's Writings on Art and Architecture," from Ruskin Today 1964)

Ruskin's influence extends far beyond the field of art history. The author Leo Tolstoy described him as "one of those rare men who think with their heart." Marcel Proust was a Ruskin enthusiast and translated his works into French. Mahatma Gandhi said that Ruskin had been the single greatest influence in his life.