McDermott has a younger sister, Robin. He 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". 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. Having met on-set in early 2014, on January 14, 2015, it was announced he was engaged to his "Stalker" co-star Maggie Q, though the couple stated, in 2017, that they are not in any rush to have an actual wedding ceremony. As of February 2019, the couple have split after a four-year engagement.