Cumming is openly bisexual. He lives in Manhattan with his husband, graphic artist Grant Shaffer, and their dogs, Honey and Leon. The couple dated for two years before entering into a civil partnership at the Old Royal Naval College in Greenwich, London, on January 7, 2007. Cumming and Shaffer legally married in New York on 7 January 2012, the fifth anniversary of their London union.
Previous relationships include an eight-year marriage to actress Hilary Lyon, a two-year relationship with actress Saffron Burrows, and a six-year relationship with theater director Nick Philippou. In 2006, Cumming stated that he “would dearly like to adopt a child, “but that his life was “too hectic” for children.