McCrory was originally cast as Bellatrix Lestrange for Harry Potter and the Order of the Phoenix, but was forced to pull out of the role when she became pregnant after which she was replaced by Helena Bonham Carter. McCrory now plays Bellatrix's younger sister instead. She later appeared in Doctor Who as the human Rosanna Calvierri, and is one of several actors to appear in both the Harry Potter film series and Doctor Who. Helen lives with her husband, actor Damian Lewis, and their two children, Manon and Gulliver, in Tufnell Park, London.
Her notable film and television roles include Aunt Polly in Peaky Blinders, Madame Kali in Penny Dreadful, Amanda Puryer on five episodes of Life (which starred Lewis, her real-life husband, in the lead role) and Mrs Vandeleur, the American fiancee of Ian Hart's Dr Watson in Sherlock Holmes and the Case of the Silk Stocking.