Seymour-Smith was born in London and educated at Highgate School and St Edmund Hall, Oxford, where he was editor of Isis. 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 (whom he met first at age 14 and maintained close ties with), 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 (1969) and Sex and Society (1975) 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 (1972), from their schooldays at Highgate School and the two often swapped notes.
Seymour-Smith's Poets Through their Letters Vol 1 (Wyatt to Coleridge) was acclaimed for its scholarship, but sold poorly. Hence, Volume 2 was never published.
His two volumes of poetry Tea with Miss Stockport (1963) and Reminiscences of Norma (1971), were praised by many, including Peter Porter. But an apparent creative silence till his last collection, Wilderness (1994), led to a decline in his reputation with the reading public during the 1980s.
The Guide to Modern World Literature is an encyclopedic attempt to describe all major 20th-century authors, in all languages. The book is over 1450 pages long. Cyril Connolly said of the first (1973) 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 (1951–76) 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 underrated, 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 overrated.
- The Guide to Modern World Literature, Hodder & Stoughton, London (1975)
- Who's Who in 20th Century Literature, Mcgraw-Hill, Columbus, OH. (1977) ISBN 0-07-056350-0
- Novels and Novelists: A Guide to the World of Fiction, St. Martins Press, London (1980) ISBN 0-312-57966-7
- A Reader's Guide to Fifty European Novels, Rl Innactive Titles (1980) ISBN 0-389-20138-3
- Robert Graves: His Life and Work, Bloomsbury Publishing PLC., London (1982) ISBN 0-7475-2205-7
- The New Astrologer, Macmillan Pub Co., London (1983) ISBN 978-0-02-081940-0
- The New Guide to Modern Literature, Peter Bedrick Books, New York (1985) ISBN 0-87226-000-3
- The 100 Most Influential Books Ever Written, MJF Books - Fine Communications, New York (1998) ISBN 1-56731-678-6
- Collected Poems 1943-1993, Greenwich Exchange (2006)
- Jenner, Simon. Martin Seymour-Smith. Obituary. "In 1981, he had been a student of astrology for more than twenty-five years when he published his only astrology book, “The New Astrologer.”"  (retrieved 17 August 2011)
- Mark Wormald "Seymour-Smith, Martin" in Ian Hamilton (ed) The Oxford Companion to Twentieth Century Poetry, Oxford: Oxford University Press, 1994, p.487
- Nye, Robert. Obituary: Martin Seymour-Smith, The Independent (1998)
- Ebooks and Martin Seymour-Smith by Robert Nagle. A writer envisions Seymour-Smith's classic New World of Modern Literature as an Ebook. Contains generous quotes.
- Simon Jenner. Biography of Martin Seymour-Smith
- Robert Nye. Obituary: Janet Seymour-Smith, The Independent, 16 September 1998 (she died 2 months after her husband)