Mercedes McNab has been acting professionally since she was 10 years old. Born and raised in Vancouver, British Columbia, Canada, she moved with her parents to Los Angeles at age nine where she had a desire to become an actress and soon landed an agent.

Her first role, at age 10, is probably the one where she is most widely recognized for in a bit part as the young girl scout in the hit film, The Addams Family. She even landed a similar role as Amanda in the sequel, Addams Family Values. But the role she is most famous for is a recurring role in playing the dim-witted Harmony Kendall on Buffy the Vampire Slayer for four years, and later its spin-off Angel: The Series 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, and appearances in Harry and the Hendersons, Touched by an Angel and Walker, Texas Ranger.
Her father Bob McNab was a member of Arsenal Football Club's league and FA Cup winning side of 1971. Auditioned for the role of "Buffy" on Buffy the Vampire Slayer but lost out to Sarah Michelle Gellar. Like a number of other actresses who tried out for the part (Charisma Carpenter (Cordelia), Julie Benz (Darla) & Elizabeth Anne Allen (Amy)), she was given another role on the series, Harmony Kendall. The character made 15 guest appearances on the show over 5 years and played a pivotal role on the spinoff, Angel: The Series. Became a regular character and was added to the credits in _"Angel" (1999)_ on her 12th appearance on the show. After having returned to school from 2001 to 2003, she rejoined the acting world full time and straight away found herself with the cast of Angel: The Series in season five in her long-running role of Harmony Kendall. Is an only child. Shares a birthday with Taylor Hanson, Kylie Tyndall and Keaton Tyndall, Quincy Jones, Chris Klein, & Michael Caine Voted one of the 25 sexiest Celebrities by Playboy Magazine (February 2007).