Grosz has worked as both an actor and as a producer. He is most recognizable for his "two guys" Sonic commercials. Grosz is a graduate of Northwestern University, where he worked with Seth Meyers of Saturday Night Live. He worked as a writer for The Colbert Report from 2007 to 2010, and once on May 13, 2010 to interrupt Stephen Colbert's attempt to introduce guest band The Hold Steady, only to perform the introduction himself. In addition, the May 13, 2010 episode ended with Colbert wishing farewell to the Time-Travelling Brandy Thief; Grosz confirmed on his Facebook page that this was his last episode of the Colbert Report as a writer. Additionally the birth of his child was announced on the show on March 9, 2009.
Peter has also appeared as a panelist and guest host on the NPR show "Wait Wait... Don't Tell Me".