Mortimer starred in Daniel Barber's first film, "Harry Brown" (2009), as police detective Alice Frampton. The plot follows widowed veteran (Michael Caine), who takes the law into his own hands when teenage violence ruin his community. Mortimer chose the role because "It felt like unfamiliar territory for me which is always a challenge and interesting [...] the script is very gripping and brilliantly written [...] getting to work with Michael, and then just the character being in some ways both the opposite and the same as Michael's character." To prepare for the role, she spent time with a real female detective, and learnt about police interrogation techniques. Upon release, the critic from "USA Today" thought her performance was "caring and savvy", despite the film's "senseless violence". Betsy Sharkey of "Los Angeles Times" wrote of Mortimer's performance: "Her clinical coolness plays well off of Caine’s controlled heat."