She played "Annette Nicholls" in the 2010 three-part TV series "Five Daughters". She appeared as Detective Superintendent Sarah Cavendish in the ninth, and final, series of "Waking the Dead". In 2011, Birthistle appeared in the Sky1 TV series "" as Captain Kate Marshall. In 2013, Birthistle was co-starring Anna Friel in the Sky1 TV production "The Psychopath Next Door". In "Brooklyn" she played Georgina, the cabin-mate of Alish (Saoirse Ronan), mentoring her in surviving the voyage to New York and dealing with immigration to the USA. In 2014, she portrayed Sarah Bailey in the miniseries "Amber". Since 2015, she has starred in "The Last Kingdom" as Hilde, a nun turned warrior and friend of Uhtred of Bebbanburg.