Walsh appeared on an episode of Veronica Mars and also had a minor role in the film Disturbia. She portrayed Katie in the original pilot for the sitcom The Big Bang Theory, but was replaced by Kaley Cuoco (as Penny) in the second pilot. In 2007 she had a role in the ABC movie Full of It as Vikki Sanders. In 2008, Walsh had a role in the Internet series The Remnants with Ze Frank. She can be seen in the 2009 comedy film Ghosts of Girlfriends Past. In 2010, she appeared in ABC Family's Beauty and the Briefcase playing Hilary Duff's best friend Joanne. In 2011, Walsh appeared in an episode of the Showcase series Single White Spenny as well as a minor appearance on the ABC crime drama Castle. She has also appeared in season 10, episode 5 of Two and a Half Men as Dawn, in season 10, episode 3 of NCIS as Ellen Roberts, and in season 1, episode 12 of The Mob Doctor as Chloe. In 2014–15, she appeared in 10 episodes of Lost Girl as Elizabeth Helm/Zee. In 2017, she appeared in the second season of Dirk Gently's Holistic Detective Agency in a central role of Suzie Boreton.