Stewart's theatre credits include: Noël Coward's Present Laughter (2007–2008) and The Hour We Knew Nothing Of Each Other at the National Theatre, London, Natalya Petrovna in Ivan Turgenev's A Month in the Country for the RSC, The Pain and the Itch (2007) at the Royal Court, London, and Proof (2002) at the Donmar Warehouse, London.

She holds dual citizenship for the UK and the US. Now based in Twickenham, she was married to actor Aden Gillett. They have two children, son Sam and daughter India. She was later in a relationship with actor Mark Powley. She had surgery for breast cancer in 2011, and advocates for causes that promote awareness of and fighting the disease.

On television, she has starred in programmes such as: Taggart in 1992 as Alison Bain, a TV Journalist with Mark McManus in Ring of Deceit, Wire in the Blood, Life Begins, Monarch of the Glen, Rebus, NCS: Manhunt, Auf Wiedersehen, Pet as Heather Lane, Midsomer Murders, as Carolyn Armitage, Mayo, as Hope Hendrick, Holby City as Kathryn MacKenzie, A Touch of Frost as Martine Phillips, New Tricks as Lulu Questor, The House of Eliott as Francine Bailey. She appeared as 'Susie' in a 1993 episode of Minder. Stewart played the role of the Production assistant, Jenny, in the first series of Drop the Dead Donkey. She also appeared in a single episode of the BBC hit comedy Men Behaving Badly as a friend of Deborah's, in the episode of Waking the Dead, Towers of Silence and in the episode of Hetty Wainthropp Investigates Season 3, "Helping Hansi" (1998). Sara played the Sheriff of Nottingham's sister, Davina, in Sister Hood, the opening episode of Season 2 of Robin Hood (2007). Her most recent roles were as Stella in the hit 2005 TV series, Sugar Rush, and as a transgender character Gaynor in an episode of the BBC drama Ashes to Ashes. She has been seen at Patricia in the Sky1 adaptation of Martina Cole's 'The Take'. In 2011 and 2012 Stewart played Professor Jean Shales in series 1 and 2 of the Channel 4 comedy series Fresh Meat. Her voice appears in Dr Who in the episode "The End of the World" In 2014 she appeared in Quirke as Rose. On 13 March 2014 she appeared in the British television Soap EastEnders as the Queens Counsel in court for 1 episode. She appeared as American Journalist Winifred Bonfils Black in ITV's Mr Selfridge.

In 2018, she appeared as Professor Ariane Cornell in an episode of Casualty. An associate of Connie Beauchamp, Cornell is involved in removing a cancerous tumor from her brain and saving her life. Stewart later appeared as Cornell in an episode of Holby City.