Sommer has a regular supporting role on AMC's television series Mad Men as Harry Crane, a media buyer who becomes head of the television department in a 1960s Madison Avenuead agency. He can be seen in ImprovEverywhere's viral video "Frozen Grand Central" around 0:38 in the clip, and on September 25, 2008, Sommer appeared as a guest star on the television series The Office. On August 7, 2011, Sommer guest starred as a veterinarian on Curb Your Enthusiasm, and in May 2012, he began performances in the Roundabout Theatre's production of Harvey on Broadway.
He lives in Los Angeles with his family. He and his wife Virginia have a daughter, Beatrice (born 2008), and a son, Patrick Ryan (born August 31, 2010). He maintained a blog at Vox, which he began using as a beta tester. He is a keen board gamer.