Malcomson starred as Trixie in the HBO series Deadwood and Colleen in ABC's Lost. She played the role of Amanda Graystone in the Battlestar Galactica spin-off series Caprica, on the Sci Fi Channel, as well as the role of Maureen Ashby on the FX Series Sons of Anarchy. She played the long-suffering Abby Donovan, wife of the title character in the Showtime series Ray Donovan.
In March 2011, Malcomson guest starred in the Fringe episode "Stowaway". She played Mrs. Everdeen in the film adaptation of The Hunger Games, her first major starring movie role.