Swallow's first television role was in Guiding Light, and she later played parts in Southland, Ringer, The Good Wife, NCIS, Flight of the Conchords, Medium, as series regular Dr. Michelle Robidaux on TNT's medical drama Monday Mornings and Rizzoli & Isles. She had a starring role in The Mentalist as FBI agent Kim Fischer. In 2015 she was cast in the eleventh season of Supernatural as the new character Amara, "the Darkness".