The second season of the Aaron Sorkin drama will revolve around the recent 2012 presidential campaign. Zimmer, who previously starred in "Entourage," will play Taylor, a press spokesperson for the Mitt Romney campaign.
The 42-year-old actress plays Washington Herald reporter Janine Skorsky on David Fincher's Netflix political drama "House of Cards." She also has a recurring role on the current season of ABC's "Grey's Anatomy" as Dr. Alana Cahill.
Her other TV credits include roles in "Royal Pains," "The New Normal," "Boston Legal" and the short-lived sitcom "Good Morning, Miami."
"Newsroom" also recently added Oscar-winning actress Marcia Gay Harden to replace Rosemarie DeWitt as lawyer Rebecca Halliday, who defends Atlantic Cable News in a wrongful termination lawsuit.
Other guest stars will include Grace Gummer as a political reporter and comedian Patton Oswalt, who will portray the new vice-president of human resources at ACN.
"The Newsroom" Season 2 is slated to premiere in June on HBO.