After reading English at Liverpool University, Thomas gained national attention when her play, Shamrocks And Crocodiles, won the John Whiting Award in 1985. Her play Indigo was performed by the Royal Shakespeare Company in their 1987/88 season. Other theatrical work includes Some Singing Blood at London's Royal Court Theatre, and an adaptation of Ibsen's The Lady from the Sea, presented in London and at the National Theatre of Norway in Oslo. Her play The House of Special Purpose was staged at the Chichester Festival Theatre in 2010.
Her screen adaptations include feature film I Capture the Castle (2003)  and the screenplay for a BBC television adaptation of Madame Bovary (2000). In 2007 she was the creator, writer and executive producer of BBC period drama Lilies. She wrote the screenplays for two major BBC adaptations of Elizabeth Gaskell's Cranford, and a film adaptation of the Noel Streatfeild novel Ballet Shoes.
In 2010 she was writer and executive producer of a major revival of the classic British television drama series Upstairs, Downstairs for the BBC. In 2011 she wrote and co-produced an adaptation of the Call the Midwife trilogy of books by Jennifer Worth. The TV series Call the Midwife began transmission in January 2012 on BBC1, achieving record viewing figures for the channel. A second and third series aired in subsequent years. In February 2014 BBC announced that a 2014 Christmas Special and a fourth series had been commissioned.  
In March 2008, she received the Best Writer award at the UK Royal Television Society awards for her work on Cranford. In April 2008 she received the Best Writer award at the UK Broadcasting Press Guild Awards for her work on Cranford, Ballet Shoes, and Lilies. She was nominated for two BAFTA TV Awards for Cranford as well as a Primetime Emmy. In November 2008 she received the Writers' Guild of Great Britain award for Best TV Series for Cranford. In 2011 she received a Primetime Emmy nomination for Upstairs Downstairs. In December 2012, the annual UK 'Women in Film and Television' awards presented her with the Technicolor Writing Award in recognition of her contribution to the industry.
A pre-Broadway production of the musical Gigi is newly adapted by Thomas and is planned to run at the Kennedy Center in January 2015.
- Madame Bovary, 2000, BBC One
- I Capture the Castle, 2003, BBC Films
- Lilies, 2007, BBC One
- Ballet Shoes, 2007, BBC One
- Cranford, 2007, BBC One
- Cranford, 2009, BBC One
- Upstairs Downstairs, 2010, BBC One
- Call the Midwife, 2012, BBC One
- All Flesh is Grass, 1984, National Youth Theatre
- Indigo, 1988, Royal Shakespeare Company and Almeida Theatre, London
- Shamrocks and Crocodiles, 1987, Liverpool Playhouse and National Theatre Studio
- Some Singing Blood, 1992, Royal Court Theatre
- The House of Special Purpose, 2009, Chichester Festival Theatre
- "Ballet Shoes press release". bbc.co.uk. Retrieved 2007-12-02.
- "Cranford - BBC official website". BBC. Retrieved 2007-11-19.
- "Lilies by Heidi Thomas - a new drama series for BBC One". BBC Press Office. Retrieved 2007-04-12.
- "Call the Midwife press release". bbc.co.uk. Retrieved 2011-01-13.
- "Danny Cohen orders eight new episodes of Call The Midwife - BBC One’s biggest new drama launch on record". bbc.co.uk. Retrieved 2012-01-24.
- "Heidi Thomas". doollee. Retrieved 2007-04-12.
- Heidi Thomas at the Internet Movie Database
- Q&A: Off Script - Heidi Thomas[dead link] – Q&A from BBC website