Thomas was born 21 October 1931 in Windsor, England, to Hugh Whitelegge Thomas, a colonial commissioner, and his wife Margery Augusta Angelo, née Swynnerton.  Sir Shenton Thomas was his uncle.  He was educated at Sherborne School in Dorset, before taking a BA in 1951 at Queens' College, Cambridge,  where he was a major scholar and was later an Honorary Fellow. Thomas gained a first class in Part I of the History Tripos in 1952 and was president of the Cambridge Union Society in 1953. He also studied at the Sorbonne in Paris.
From 1954 to 1957, Thomas worked in the Foreign Office partly as secretary of the British Delegation to the sub-committee of the UN Disarmament Commission. From 1966 to 1975, he was professor of History at the University of Reading, and chairman of the European committee. Thomas was then chairman of the Centre for Policy Studies in London from 1979 to 1991, which worked for Prime Minister Margaret Thatcher.
Until 1974, Thomas was a member of the Labour Party.  He became a life peer as Baron Thomas of Swynnerton, of Notting Hill in Greater London in letters patent dated 16 June 1981 and sat as a Conservative, before he joined the Liberal Democrats in 1998.  He later sat as a crossbencher.
He wrote political works favouring European integration such as Europe: the Radical Challenge (1973), as well as histories. He was also the author of three novels: The World's Game (1957), The Oxygen Age (1958), and Klara (1988). Thomas' 1961 book The Spanish Civil War won the Somerset Maugham Award for 1962. A significantly revised and enlarged third edition was published in 1977; further editions were published in 1999 and 2012. 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. In 1985, he signed a petition against the political project of the Sandinista National Liberation Front of Nicaragua.  and in support for the Contras, an anti-Sandinista paramilitary group.
Thomas was married to Hon. Vanessa Jebb, a painter and daughter of Gladwyn Jebb, the first Acting United Nations Secretary-General and British Ambassador to France. They had three children: Inigo, Isambard and Isabella. 
Thomas won the Somerset Maugham Award (1962), the Nonino Prize (2009), the Boccaccio Prize (2009), the Gabarrón Prize (2008) and the Calvo Serer Prize (2009). The French Government appointed him Commander of the Order of Arts and Letters in 2008.
Thomas also received the Grand Cross of the Royal Order of Isabella the Catholic  from Spain as well as the Mexican Order of the Aztec Eagle, the Joaquín Romero Murube Prize in Seville (2013) and the Grand Cross of the Civil Order of Alfonso X the Wise (2014). 
- Disarmament – the way ahead Fabian Society (1957).
- The Spanish Civil War (1961); Penguin Books Ltd (1968); 2nd Revised edition (1977); 4th Revised edition (2003). A new revised edition in 2011  commemorated the book being 50 years in print; it was published in 15 languages.
- Cuba or the Pursuit of Freedom (1971); revised editions (1998), (2002), (2010).
- Goya: The Third of May 1808 (1972).
- Europe: the Radical Challenge (1973).
- John Strachey (1973).
- An Unfinished History of the World (1979); published in the United States as A History of the World, then as "World History" (1998); and by the original title in London (by Hamish Hamilton) in (1979) and with revised editions in (1981) and (1982).
- The revolution on balance (1983), Washington, DC; Cuban American National Foundation 1983 (CANF pamphlet #5).
- Armed Truce (1986). A history of the beginning of the Cold War.
- Ever Closer Union (1991).
- The conquest of Mexico (1993); published in USA as Conquest: Montezuma, Cortés and the Fall of Old Mexico.
- The Slave Trade: The History of the Atlantic Slave Trade 1440–1870 (1997); Simon and Schuster.
- Who Is Who of the Conquistadors (2000). A study of those who fought for Cortés.
- Rivers of Gold (2003); first book of a trilogy about the Spanish Empire.
- Beaumarchais in Seville (2006); ISBN 978-0-300-12103-2.
- Eduardo Barreiros and the Recovery of Spain (2009); a biography of Eduardo Barreiros.
- The Golden Age: The Spanish Empire of Charles V (2010); second book of a trilogy about the Spanish Empire. Published in USA as The Golden Empire: Spain, Charles V, and the Creation of America (2011).
- The World's Game; a novel (1957).
- The Oxygen Age; a novel (1958).
- Klara, a novel (1988).
- The Suez Affair (1966); an analysis of the Suez Crisis of 1956.
- World Without End: The Global Empire of Philip II (2014); third volume of a trilogy about the Spanish Empire.
- Preston, Paul (9 May 2017). "Lord Thomas of Swynnerton". The Guardian. Retrieved 10 May 2017.
- Guimón, Pablo (8 May 2017). "Obituary: Hugh Thomas, author of seminal book on the Spanish Civil War". El País. Retrieved 9 May 2017.
- "Hugh Thomas (Baron Thomas of Swynnnerton)". Portal del hispanismo (in Spanish). Archived from the original on 20 July 2011. Retrieved 8 May 2017.
- Cowell, Alan (11 May 2017). "Hugh Thomas, Prodigious Author of Spanish History, Dies at 85". The New York Times. p. B14. Retrieved 15 May 2017.
- "Former Head Of Conservative Think-Tank Joins The Liberal Democrats /Pr Newswire Uk/". Prnewswire.co.uk. Retrieved 2017-01-15.
- "No. 48657". The London Gazette. 19 June 1981. p. 8253.
- "Quand Bernard-Henri Lévy pétitionnait contre le régime légal du Nicaragua". Monde-diplomatique.fr. 1 October 2009. Retrieved 10 December 2017.
- "Otras disposicions" (PDF). Boletín Oficial del Estado (in Spanish). 7 April 2001. Archived from the original on 29 December 2014. Retrieved 8 May 2017.
- "Otras disposiciones" (PDF). Boletín Oficial del Estado (in Spanish). 27 December 2014. Archived from the original on 29 December 2014. Retrieved 8 May 2017.
- The Spanish Civil War: Revised Edition (Modern Library Paperbacks) 'Modern Library', ISBN 978-0-375-75515-6