Around 1995, O'Quinn made guest appearances in "The X-Files" and "Harsh Realm", produced by Chris Carter, who also cast him in the film "The X-Files: Fight The Future" and then once again in the final season. In 1996 O'Quinn started acting in the television series "Millennium" as Peter Watts, also produced by Chris Carter. O'Quinn held this role for all three seasons of the series. O'Quinn holds the distinction of having played four different characters within the extended "X-Files"/"Millennium" continuum (the two shows being classed together since both Lance Henriksen's character of Frank Black and Charles Nelson Reilly's character of Jose Chung have appeared in both shows). O'Quinn made his breakthrough by starring as the deranged serial-killing title character in "The Stepfather". He was praised by critics, including Roger Ebert in the "Chicago Sun-Times", who commented: "'The Stepfather' has one wonderful element: Terry O'Quinn's performance". Ebert wrote: "He is a journeyman actor from TV and many movies, usually in supporting roles, and you may or may not recognize him. What's clear at once is that he is a strong actor, and given this leading role he brings all kinds of creepy dimensions to it. He has the thankless assignment of showing us a completely hateful, repellent character – and he approaches the task as an exercise in cloying middle-class good manners." O'Quinn was nominated for both a Saturn Award and an Independent Spirit Award for his performance. In the film, O'Quinn plays a deranged serial killer going by the name "Jerry Blake" (his character's real name and identity are never revealed), who is obsessed with having an ideal family. When a widowed mother and daughter do not comport with his expectations, he spirals into a spell of madness and attempts to brutally murder them. A sequel was released two years after the first film, in which his character escapes from the asylum in which he is placed, and steals the identity of a man named Gene Clifford after reading his obituary in a local newspaper. It grossed almost a million dollars less at the box office. Not impressed with the second movie, O'Quinn declined to appear in the third installment, in which the stepfather character was portrayed by Robert Wightman. O'Quinn was approached by director of the 2009 reboot of "The Stepfather", Nelson McCormick, to make a cameo appearance in the film, but according to the producers O'Quinn turned down the offer.