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 series achieved 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.
A fifth series was commissioned for 2016, shortly after series four was done filming. On 23 November 2016, the BBC announced a three-year deal with Neal Street Productions, commissioning a seventh, eighth and ninth series, all with Christmas specials. The new commission will keep the series on air until at least 2020.
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.
- 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 Christmas Special, 2009, BBC One
- Upstairs Downstairs, 2010, BBC One
- Call the Midwife, 2012–present, BBC One
- Little Women, 2017 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 2 December 2007.
- "Cranford - BBC official website". BBC. Retrieved 19 November 2007.
- "Lilies by Heidi Thomas - a new drama series for BBC One". BBC Press Office. Retrieved 12 April 2007.
- "Call the Midwife press release". bbc.co.uk. Retrieved 13 January 2011.
- "Danny Cohen orders eight new episodes of Call The Midwife - BBC One's biggest new drama launch on record". bbc.co.uk. Retrieved 24 January 2012.
- "Midwives to return for a fourth series". 10 March 2014. Retrieved 29 January 2018 – via www.BBC.co.uk.
- "Call the Midwife gets a fourth series". Radio Times.
- Ellie Walker-Arnott (1 April 2016). "Call the Midwife reveals first look at 2016 Christmas special". Radio Times. Retrieved 9 November 2016.
- Mitchell, Bea. "Call the Midwife is coming back... for series seven, eight AND nine". Digitalspy.com. Retrieved 1 February 2017.
- Gigi Archived 17 May 2014 at the Wayback Machine Official Site
- Guild, Writers' (14 January 2019). "Writers' Guild Award winners 2019". writersguild.org.uk. Retrieved 20 January 2019.
- "Dr Heidi Thomas dominates festive TV". edgehill.ac.uk. 22 December 2017. Retrieved 20 July 2019.
- "Call The Midwife duo receive honorary degrees". liverpool.ac.uk. 19 July 2019. Retrieved 20 July 2019.
- "Heidi Thomas". doollee. Retrieved 12 April 2007.