Thomas first gained national attention when her play, Shamrocks And Crocodiles, won the John Whiting Award in 1985.

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. She has also written episodes for television series Soldier Soldier and Dr. Finlay. Forthcoming work includes a major revival of the classic British television drama series Upstairs Downstairs for the BBC.

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.

Screenplays

• 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

Plays

• 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