She appeared as Camilla "Chummy" Fortescue-Cholmondeley-Browne in the BBC drama series Call the Midwife (2012–2015) and made her Hollywood debut in the action comedy film Spy (2015).

[10] Her father was commanding officer of HMS Coventry when it was sunk during the 1982 Falklands conflict, and he was badly burned trying to escape the stricken warship.

She also appeared in even smaller roles in French & Saunders, My Family and Other Animals, Nighty Night, Absolutely Fabulous, The Vicar of Dibley as a speed-date programmer, Lead Balloon, William and Mary, Smack the Pony (for which she wrote and performed in a couple of sketches and a number of video diaries), Stupid!, Monday Monday as Tall Karen, and as a minicab driver in the Channel 5 comedy Angelos, which ran for 6 episodes.

Hart is best known for her performance in her self-titled, self-penned and semi-autobiographical sitcom Miranda, which started recording for BBC Two in 2008 and began airing on 9 November 2009.

[18] The situation comedy also features Sarah Hadland, Tom Ellis, Patricia Hodge, James Holmes, and Sally Phillips.

[18][19] Described as an "old-fashioned" sitcom, it received positive comments from critics and Hart won the 2009 Royal Television Society award for comedy performance for her role in the first series.

[22][23] In 2012, Hart began to appear in the BBC One drama Call the Midwife,[24] playing the character of "Camilla 'Chummy' Fortescue-Cholmondeley-Browne".

[26] Also, in December 2013, she was lined up to star in To Love, Honour and Betray (Till Divorce Do Us Part), the television version of Kathy Lette's novel.

"[29] A 10th anniversary special of Miranda, filmed at the London Palladium in 2019 and described by Hart as "a party (not a new episode)",[30] aired on BBC One on 1 January 2020.

Hart made a cameo appearance as a loan officer in the 2007 comedy film Magicians which featured David Mitchell and Robert Webb, both stars of the long running television series Peep Show.

[34] In 2020, Hart played "harmless chatterbox" Miss Bates in the Jane Austen adaptation film Emma.,[a] alongside Anya Taylor-Joy and Bill Nighy.

Jon Holmes is a highly experienced presenter from BBC Radio 6 Music [...] BBC Radio 2 appreciates if their presentation wasn't to everyone's liking, but feels it's important to be able to bring new talent to its output and hopes its audience understands the importance of maintaining a breadth of content on the network.

[42] Hart embarked on her first tour in 2014, titled My, What I Call, Live Show, performing in arenas in the UK and Ireland.

[44] Two years later, she appeared in the final sketch from comedy duo French and Saunders, which was broadcast during Red Nose Day 2009.

In 2010, she and six other TV celebrities raised over £1 million for the charity Sport Relief cycling from John O'Groats to Land's End.

In aid of Comic Relief, a Miranda mini-episode set in the world of Pineapple Dance Studios was broadcast on 18 March as part of Red Nose Day 2011.

[48] Hart released a book in October 2012 titled Is It Just Me?, featuring stories, anecdotes and life advice delivered in her own style.

[53] In her early twenties, Hart had an unsuccessful trial for Queens Park Rangers' women's team; she revealed this during Would I Lie to You.

[57] In her memoir published in October 2024, I Haven't Been Entirely Honest With You, she revealed that she had been diagnosed with Lyme disease, which led to her becoming bed and house-bound with ME/CFS.

On 5 January 2025 Hart's life and work, in relation to her tastes and experiences of music, was the subject of BBC Radio 3's programme Private Passions, presented by Michael Berkeley.