Llewellyn was born in Hendon, Middlesex in 1906,  the oldest of three children to Welsh parents Sarah Anne née Thomas (1878–1929) and William Llewellyn Lloyd (1876–1942), a hotel clerk and later the assistant secretary to a club. Only after his death was it discovered that Llewellyn's claim that he was born in St Davids, West Wales, was false.
He lived a peripatetic existence, travelling widely throughout his life. Before World War II he spent periods working in hotels, wrote a play, worked as a coal miner and produced his best-known novel. During World War II he rose to the rank of Captain in the Welsh Guards. Following the war he worked as a journalist, covering the Nuremberg Trials, and then as a screenwriter for MGM. Late in his life, he lived in Eilat, Israel.
Llewellyn married twice: his first wife was Nona Theresa Catherine Sonsteby (1921&ndsh;1988), whom he married in 1952 and divorced in 1968; his second wife was Susan Heimann, whom he married in 1974.
Richard Llewellyn died in Dublin on 30 November 1983.
Several of his novels dealt with a Welsh theme, the best-known being How Green Was My Valley (1939), which won international acclaim and was made into a classic Hollywood film. It immortalised the way of life of the South Wales Valleys coal mining communities, where Llewellyn spent a small amount of time with his grandfather. Three sequels followed.
Llewellyn's novels often included the recurring element of protagonists who assume new identities (as they are transplanted into foreign cultures), such as the character Edmund Trothe whose adventures extend through several spy adventure books.
- Poison Pen: A Play in Three Acts (1938)
- How Green Was My Valley (1939)
- None but the Lonely Heart (1943)
- A Few Flowers for Shiner (1950)
- A Flame for Doubting Thomas (1954)
- Sweet Witch (1955)
- Mr. Hamish Gleave (1956)
- The Flame of Hercules (1957)
- Warden of the Smoke and Bells (1958)
- Chez Pavan (1959)
- Up into the Singing Mountain (1960)
- A Man in a Mirror (1964)
- Sweet Morn of Judas' Day (1965)
- Down Where the Moon is Small (1966)
- Bride of Israel My Love (1973)
- Hill of Many Dreams (1974)
- Green, Green My Valley Now (1975)
- At Sunrise, the Rough Music (1976)
- Tell Me Now and Again (1977)
- A Night of Bright Stars (1979)
- I Stand on a Quiet Shore (1982)
- Edmund Trothe series
- "True birthplace of Wales's literary hero". BBC News. BBC. 1999-12-05. Retrieved 2010-04-06.
- Ruthven, KK (2001). Faking Literature. Cambridge University Press. p. 13. ISBN 0-521-66015-7.
- "BBC Wales – Arts - Richard Llewellyn". BBC Cymru Wales website. BBC Cymru Wales. 28 November 2008. Retrieved 2010-04-05.
- "Lloyd, Richard Dafydd Vivian Llewellyn (1906–1983)". Oxford Dictionary of National Biography. Sep 2004. Retrieved 2010-04-11.
- England & Wales, Civil Registration Birth Index, 1837-1915 for Richard Herbert V Lloyd, 1907, Q1-Jan-Feb-Mar: Ancestry.com (subscription required)
- 1911 England Census for William Llewellyn Lloyd, Middlesex, Tottenham, West Tottenham: Ancestry.com (subscription required)
- Llewellyn, Richard, 1906-1983 Archived 2013-12-12 at the Wayback Machine
- What appears to be his birth certificate gives his name as Richard Herbert V Lloyd, see http://www.freebmd.org.uk
- "Books and Authors", The New York Times, 16 February 1941, page BR12. ProQuest Historical Newspapers The New York Times (1851–2007).
- England & Wales, Civil Registration Marriage Index, 1916-2005 for Richard D V L Lloyd, 1974: Ancestry.com (subscription required)