Went to boarding school in Concord, NH, when he was 13.
Worked as a dishwasher and chef after high school while deciding how to pursue acting.
Moved to London, England, at 17 to study acting at the London Academy of Music and Dramatic Art.
One of his first showbiz jobs was appearing in the music video for George Harrison's "Got My Mind Set on You."
Appeared in a 1993 British stage production of Rope with future Buffy the Vampire Slayer costar Anthony Head. Years later, he ran into Head at an L.A. book signing. The reunion led Head to recommend Denisof for the part of Wesley Wyndam-Pryce, the rival Watcher to Head's Rupert Giles in Season 3.
Relocated to L.A. after 13 years in London, but had difficulty finding work. Mary Steenburgen, whom he had met while filming Noah's Ark, and her husband Ted Danson put him up for a year.
Met wife Alyson Hannigan when he appeared on Buffy the Vampire Slayer, though they didn't begin dating until after he had moved on to the Buffy spinoff Angel.