McDermott married actress Shiva Rose on November 19, 1995. They have two daughters, Colette and Charlotte. Colette's birth is prominently featured in Ensler's The Vagina Monologues. Colette graduated from Barnard College, of which McDermott was also a trustee. On September 27, 2007, People confirmed that McDermott and Rose had separated. On May 16, 2008, it was reported that McDermott had filed for divorce from Rose. The divorce was finalized on January 2, 2009.
