He is also known for his roles in the comedy-drama films The Devil Wears Prada (2006), Celeste and Jesse Forever (2012), The Giant Mechanical Man (2012), Hello, My Name Is Doris (2015), and BlackBerry (2023) as well as voicing Henry in the 2016 video game Firewatch.

From 2007 until its ending in 2015, Sommer maintained a regular role on AMC's period drama series Mad Men as Harry Crane, a media buyer who becomes head of the television department in a 1960s Madison Avenue ad agency.

[7] In 2011, Sommer guest starred in two episodes of The CW's drama-thriller series Nikita as electrical engineer and CIA consultant Malcolm Mitchell.

[13] From February 2 to March 13, 2016, Sommer starred in the Off-Broadway revival of Sam Shepard's play Buried Child, with Ed Harris and Amy Madigan.

[14] That same year, he also co-starred in Rob Reiner's biopic LBJ as Press Secretary Pierre Salinger, alongside Woody Harrelson in the title role.