Thomas was born in Wales, in a cemetery keeper's cottage where his grandmother lived.[8] He had his first story published at nine years old in a Boy's Own Paper competition.[8] With his father in the RAF, he travelled widely and was educated at the Cairo High School, the Marist Brothers (in Port Elizabeth, South Africa) and, lastly, at Bedford Modern School.[8] His first book, completed at the age of seventeen, is the story of a British spy in Russia during the Second World War, titled Descent Into Danger.[8] He refused the offer of a job at a university in order to accompany a travelling fair for a year: he used those experiences for his novel, Bed of Nails.[8] Since then his books have been published worldwide.[9] He has been a foreign correspondent beginning with the Suez Crisis and ending with the first Gulf War. He was a BBC writer/producer for three flagship BBC programmes: Man Alive, Tomorrow's World and Horizon.[10]
He was a regular contributor to the Japanese news magazine Facta and he lectured widely on the secret world of intelligence. He also provided expert analysis on intelligence for US and European television and radio programmes.[11]
In 1998 he wrote and narrated a major documentary for Channel 4: The Spy Machine. It followed three years of research during which he was given access to Mossad's main personnel. The documentary was co-produced by Open Media and Israfilm.[12] Two years later Thomas wrote Gideon's Spies: Mossad's Secret Warriors. As Thomas's "Notes on Sources" afterword to Gideon's Spies makes clear, the research for the film provided him with some, but not all, of the sources and material he used when writing his later book independently of Channel 4 and the film-makers.[13]Gideon's Spies has so far been published in 16 languages. Sources for the book included Ari Ben-Menashe, a former Israeli intelligence agent, and Israeli spy Rafi Eitan. According to Charles Foster in Contemporary Review: "Writers who know their place are few and far between: fortunately Mr Thomas is one of them. By keeping to his place as a tremendous storyteller without a preacher's pretensions, he has put his book amongst the important chronicles of the state of Israel."[14]
Thomas was a cousin of the poet Dylan Thomas who helped him publish his first book.[8] He was once married to broadcaster Annie Nightingale. He divided his time between his homes in Ireland and England, with his wife, an interior designer.[8] His five children work in various parts of the entertainment industry.[8][18]