Himelstein is the son of Susan, née Maloney, and Robert "Bob" Himelstein; he is of Jewish heritage on his father's side and German-Irish on his mother's. He is close friends with fellow Joan of Arcadia stars Michael Welch and Chris Marquette; Himelstein and Welch had known each other for several years but did not become good friends until the series began in 2003. He dated Remember the Daze co-star Leighton Meester in 2007 and the two of them briefly rekindled their romance in 2012.