In 2008 she beat hundreds of other girls for a role in Oliver! at the Theatre Royal, Drury Lane in London. She was one of only a few girls chosen to appear in the children's ensemble and remained in the show for a year. Towards the end of her time in Oliver!, she won the role of Young Ruth in Mark Romanek's Never Let Me Go – a feature film based on the book by Kazuo Ishiguro. It was released in 2010 to positive reviews. After filming this, Ella was cast as Kayleigh in Gustavo Ron's Ways to Live Forever, adapted from the book by Sally Nicholls. She was also cast as Mia in Juan Carlos Fresnadillos Intruders, in which she plays the daughter of Clive Owen and is the central character. She was named by Screen International as one of 10 UK Stars of Tomorrow. Purnell also appeared in the BBC HD Film Short Candy in June 2011. She appeared in the feature film Kick-Ass 2, as the character 'Dolce'. She also appeared as a main character in the independent film, WildLike, and as a teenage version of the character Maleficent in Disney's 2014 film Maleficent. Purnell starred in Tim Burton's adaptation of Miss Peregrine's Home for Peculiar Children, based on the novel of the same name, by Ransom Riggs. It was released in 2016. In 2015 she filmed Access All Areas, in which she stars as Mia alongside actress Georgie Henley.The film was released in 2017. In 2017, Purnell appeared in the historical drama filmChurchill, playing Winston Churchill's secretary Helen Garrett.
TV
In 2018, Purnell appeared in the BBC mini-series Ordeal By Innocence, based on the Agatha Christiebook of the same name. Purnell played Hester Argyll, the youngest adopted child of the mysterious Argyll family. Purnell starred in the lead role of the Starz show Sweetbitter, based on Stephanie Danler's internationally best-selling novel of the same name. Purnell plays Tess, a naive 22-year-old who moves to New York City to pursue a new life, and gets caught up in the world of fine dining. After its first season aired in early 2018, Sweetbitter was renewed for a second season. It was announced in December 2019 that Starz had cancelled the series. In 2020, she features as lady Maria Grey in Julian Fellowes' Belgravia on ITV in the UK and Epix in the US.
