Fox began the 1990s with roles in TV productions including the specials "Summer Stories: The Mall", an installment of "ABC Afterschool Specials" and "Dead Drunk: The Kevin Tunell Story", one of HBO's "Lifestories: Families in Crisis". A 1993 episode of "Law & Order" (NBC) marked Fox's transition to primetime series, where she would land a regular role as an investigator for the Chicago police on ABC's "Missing Persons" (1993-94). In 1996, Fox began her portrayal of Dr. Maggie Doyle, a gun-toting vegetarian lesbian who was in residency at the "ER" (NBC). A recurring character from 1996 through 1999, Doyle had some powerful scenes and storylines in the course of her run, and Fox handled her character with a combination of grace, reserve and vulnerability. Known to a wide audience for her portrayal of Maggie, Fox was featured in the groundbreaking 1997 "coming out" episode of ABC's "Ellen".
In 1999, Fox began a recurring role on the acclaimed NBC drama "The West Wing" playing Gina Tuscano, the Secret Service agent in charge of protecting the daughter (Elisabeth Moss) of the President of the United States (Martin Sheen). Leaving in 2000 to work full-time on "CSI", Fox and Tuscano could feasibly return to "The West Wing" thanks to an open-ended exit, but the success of "CSI" would seem to make such insurance unnecessary. Following the cases of the Las Vegas Police forensics team, the series captured an audience fascinated with the show's use of science and thrilled by the included drama. Here Fox played Sara Sidle, an inexperienced but hard-working and determined investigator hand-picked by William Petersen's Gil Grissom for the forensics team.
Though television has hosted her greatest successes to date, Fox has appeared in several films since her debut. A supporting role as Alan Arkin's love interest in the unimpressive comedy "The Jerky Boys" was one of three of the actress' big screen appearances in 1995, including black comedy "Dead Funny" and the romance "Alchemy". In 1997, in addition to a featured role in the NBC miniseries "House of Frankenstein 1997", Fox added to the less-seen but far more critically acclaimed independent feature "How to Make the Cruelest Month". A starring role in "The Hungry Bachelor's Club" came in 1999, and in 2001 she would be remembered as the murdered wife of a man suffering from anterograde amnesia (Guy Pearce) in the inventive and critically lauded feature "Memento".
In addition to film and television, Fox has worked on stage, co-founding the experimental theater group Honeypot Productions, and writing and starring in plays for the Los Angeles-based group.