McNab received her first notable acting role in 1991 when she appeared as a girl scout selling cookies in The Addams Family. She then received a more prominent role in the 1993 sequel Addams Family Values in which she portrayed a snobby camper named Amanda Buckman. McNab's next breakthrough came in 1997, when she was cast in a recurring role on the TV show Buffy the Vampire Slayer on The WB. McNab played Harmony Kendall, a vapid popular high school student who eventually becomes a vampire. She appeared in 16 episodes over four years. McNab reprised this role when she was cast in the spin-off Angel, which starred David Boreanaz. She became a series regular during the show's final season, airing from 2003-04. Since the cancellation of Angel, McNab has gone on to guest star in popular TV shows including Psych and Supernatural. In 2007, she appeared as a lead role in the horror-slasher movie Hatchet, directed by Adam Green. She later briefly appeared in the 2010 sequel Hatchet 2 and has since starred in a number of Direct-to-DVD horror movies such as Dark Reel and Thirst, as well as the TV movie Vipers alongside Tara Reid.

McNab and Mark Henderson, co-founder of the Crescent Hotel in San Francisco, wed on May 12, 2012 in La Paz, Mexico in front of family and friends. Boomkat singer Taryn Manning was among the 80 in attendance for the nuptials. The couple has one child, Vaunne Sydney Henderson, who was born on February 25, 2013.