Mercedes McNab has been acting professionally since she was 10 years old. She was born and raised in Vancouver, British Columbia, but moved with her parents to Los Angeles at age 9. She wanted to be an actress and soon landed an agent.

Her first role, at age 10, is probably her best-known: a bit part as the young Girl Scout in the hit film The Addams Family (1991). She even landed a similar role as Amanda in the sequel, Addams Family Values (1993). But the role she is most famous for is a recurring role as dimwitted Harmony Harmony Kendall on Buffy the Vampire Slayer (1997) for four years, then its spinoff, Angel (1999) in a guest spot in 2001 and later as a regular for its final season from 2003-2004.

Some of her television appearances include the unaired Fox television pilot for Buffy the Vampire Slayer (1997) and appearances in Harry and the Hendersons (1991), Touched by an Angel (1994), and Walker, Texas Ranger (1993).