Born in La Jolla, California, McKellar and sister Crystal McKellar have maintained a friendly competition to see who gets more acting jobs. In fact, when the actress who would play Winnie had to be chosen, the only two actresses who were still in competition were the McKellar sisters.
Danica and her family moved to Los Angeles when she was 8, and soon after, she and her sister began acting.
McKellar studied at UCLA, majoring in mathematics, and graduated summa cum laude in 1998. She coauthored a scientific paperTemplate:Fn which appeared in a peer-reviewed physics journal. Referring to the mathematical abilities of McKellar and her fellow student Brandy Winn, one of her math teachers, Professor Lincoln Chayes, was quoted in the New York TimesTemplate:Fn as saying, "I thought that the two were really, really first rate". McKellar uniquely even provides free online math tutoring for her fans at her official internet website.
Since then, as well as several guest roles in television series (including one with former co-star Fred Savage on "Working"), she has written and directed two short films. She has returned to regular television with a role in The West Wing.
Her acting credits include:
- The Twilight Zone (as Debbie Dobs) 1985-1986
- Volkswagen commercial, 1986
- Wide World Of Kids, (as herself), 1987-1990
- The Wonder Years, 1988-1993
- No More Rhyme (Deborah Gibson video) 1989
- Camp Cucamonga (TV movie, as Lindsey Scott) 1990
- Captain Planet (animated, voice only) 1992
- Sidekicks (feature movie, as Lauren) 1993
- Walker, Texas Ranger (played Lori in one episode) 1994
- Cradle Of Conspiracy (tv movie, she also played Lori) 1994
- Sirens, 1994
- Justice For Annie (tv movie, as Annie) 1995
- Working (Jolie in two episodes) 1997
- The Love Boat The Next Wave (played a nun in one episode) 1997
- Happy Hour (as herself) 1999
- Grease (play) (as Rizzo) 1999
- The West Wing 2002-
- Babylon 5 (in "The War Prayer" as Aria Tensus) 1994
- Template:Fnb Chayes, L., McKellar, D. & Winn, B. (1998) Percolation and Gibbs states multiplicity for ferromagnetic Ashkin–Teller models on Z. Journal of Physics A: Mathematics and General, 31, 9055–9063. Full text as PDF
- Template:Fnb Kenneth Chang, "Between Series, an Actress Became a Superstar (in Math)," New York Times, July 19, 2005.