[1] She grew up in Tolworth and Surbiton,[5] and studied at Tiffin Girls' School[6] before reading classics at St Hilda's College, Oxford.

[7][8] She then studied at the London Academy of Music and Dramatic Art, although she left before graduating in order to star in the play The Age of Consent, which premiered at the Edinburgh Festival Fringe in 2001.

While working on Doc Martin, she was asked by her friend from London Academy of Music and Dramatic Art, Chris O'Dowd to audition for the comedy series The IT Crowd as the show was having trouble casting a female lead.

[3] In 2007, she appeared in a new production of Chekhov's The Seagull at London's Royal Court Theatre, alongside Kristin Scott Thomas and Mackenzie Crook, for which she received positive reviews.

[11] At the end of 2009, she appeared in the Olivier award winning[12] play Cock at the Royal Court Theatre with Ben Whishaw and Andrew Scott.

In 2018 and 2019, she performed in Home, I'm Darling, for which she was nominated for an Olivier Award,[17] as well as in Defending the Guilty, a legal sitcom which aired for one series on BBC Two and was subsequently renewed for a second,[18] before being cancelled due to coronavirus.

[19] She played Emma Jeanne Desfosses in Marjane Satrapi's film adaptation of Lauren Redniss's Radioactive in 2019, detailing the life of Marie Curie.

[24] In 2022, Katherine played Nell Taylor in the film The Nan Movie and in 2024 she portrayed Lizzie Vereker in the Disney Plus adaptation of Jilly Cooper's Rivals (TV series) novel.