Seymour-Smith was born in London and educated at Highgate School and St Edmund Hall, Oxford, where he was editor of Isis and Oxford Poetry. His father Frank was a chief librarian who supplied books to Robert Graves, and his mother Marjorie wrote poetry and published under the name of Elena Fearn. He began as one of the most promising of Anglophone post-war poets, but became better known as a critic, writing biographies of Robert Graves, Rudyard Kipling and Thomas Hardy, and producing numerous critical studies. The poet and critic Robert Nye stated that Seymour-Smith was "one of the finest British poets after 1945." Others to praise his poetry included Robert Graves, C. H. Sisson, Geoffrey Grigson and James Reeves. He came to prominence in 1963, as the editor of the first twentieth-century edition of Shakespeare's Sonnets to use the 'original' spelling. Characteristically, his commentary concerned Shakespeare's sexuality, which upset many. Later, his Fallen Women and Sex and Society would become 'standard plundering material for more famous works' as the author good-humouredly claimed. He had known Alex Comfort, who was then writing The Joy of Sex, from their schooldays at Highgate School and the two often swapped notes. Seymour-Smith's Poets Through their Letters Vol 1 was acclaimed for its scholarship, but sold poorly. Hence, Volume 2 was never published. His two volumes of poetry Tea with Miss Stockport and Reminiscences of Norma, were praised by many, including Peter Porter. But an apparent creative silence till his last collection, Wilderness, led to a decline in his reputation with the reading public during the 1980s. The Guide to Modern World Literature is an encyclopaedic attempt to describe all major 20th-century authors, in all languages. The book is over 1450 pages long. Cyril Connolly said of the first edition: "I'm very much afraid he will prove indispensable!" His criticism of Lawrence Durrell singled out his poetry as his real achievement; John Fowles, Muriel Spark, C. P. Snow, Malcolm Bradbury and Ted Hughes received the first adverse criticism of their reputations in this book. The stature of Anthony Powell's A Dance to the Music of Time as the greatest fictional post-war achievement was asserted: a view endorsed by Kingsley Amis and Hilary Spurling. He also predicted that T. S. Eliot's Four Quartets would not survive as a great poem by 2000. The polyglot Seymour-Smith further used the book to champion writers he regarded as under-rated, such as James Hanley, Laura Riding, Wyndham Lewis, Roberto Arlt, Pio Baroja, Rayner Heppenstall and Jose Maria Arguedas, while attacking those he felt were overvalued, such as George Bernard Shaw, W. H. Auden and as mentioned above, T. S. Eliot. Seymour-Smith also disparaged Harold Pinter, Margaret Atwood, and Tom Stoppard, whom he thought over-rated. Anthony Burgess likened Seymour-Smith to Samuel Johnson due to his many literary surveys from The Guide to Modern World Literature in 1975 onwards.