Thomas was educated at Sherborne School in Dorset before taking a BA in 1953 at Queens' College, Cambridge. He gained a First Class in Part I of the History Tripos in 1952 and was President of the Union in 1953. He also studied at the Sorbonne in Paris. His 1961 book The Spanish Civil War won the Somerset Maugham Award for 1962. A significantly revised and enlarged third edition was published in 1977. Cuba, or the Pursuit of Freedom (1971) is a book of over 1,500 pages tracing the history of Cuba from Spanish colonial rule until the Cuban Revolution. Thomas spent ten years researching the contents of his book.
From 1966 to 1975 Thomas was Professor of History at the University of Reading. He was Chairman of the Centre for Policy Studies in London from 1979 to 1991, this worked for the Prime Minister Margaret Thatcher. He became a life peer as Baron Thomas of Swynnerton, of Notting Hill in Greater London in letters patent dated 16 June 1981. He has written pro-European political works, as well as histories. He is also the author of three novels.
Thomas's The Slave Trade: The Story of the Atlantic Slave Trade, 1440-1870 "begins with the first Portuguese slaving expeditions, before Columbus's voyage to the New World, and ends with the last gasp of the slave trade, long since made illegal elsewhere, in Cuba and Brazil, twenty-five years after the American Emancipation Proclamation," according to the summary on the book jacket.
- The Spanish Civil War (1961); 2nd Revised edition (1977); 4th Revised edition (2003).
- Cuba or the Pursuit of Freedom (1971)and with revised editions (1998) and (2002)
- Europe: the Radical Challenge (1973)
- An Unfinished History of the World (1979) published in USA as A History of the World (then as "World History" 1998) and as the original title in London by Hamish Hamilton in (1979) and with revised editions (1981) and (1982).
- The revolution on balance Washington, DC: Cuban American National Foundation 1983 (CANF pamphlet #5)
- Armed Truce (1986)
- Ever Closer Union (1991)
- Conquest: Montezuma, Cortés and the Fall of Old Mexico (1994)
- The Slave Trade: The History of the Atlantic Slave Trade 1440–1870 Simon and Schuster. (1997)
- Rivers of Gold (2003)
- Beaumarchais in Seville (2006) ISBN 9780300121032
- The World's Game (novel)
- The Oxygen Age (novel)
- The Suez Affair analyses the Suez Crisis of 1956
- Goya's 'The Third of May': Europe: The Radical Challenge: 1808
- Also a biography of John Strachey (London & New York 1973 and of Eduardo Barreiros, entitled "Eduardo Barreiros and the Recovery of Spain: (New Haven, 2009)
Hugh Thomas has won the Nonino Prize (2009), the Boccaccio Prize (2009), the Gabarron Prize (2008) and the Calvo Serer Prize (2009). He became a "Commander of the Order of Arts and Letters" in France in 2008 and has the Grand Cross of the Order of Isabel the Catholic in Spain as well as the Order of the Aztec Eagle in Mexico.