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.