McKellar was born in San Diego, California, but when she was seven years old, her family moved to Los Angeles. She and her sister Danica began to show interest in acting, sometimes competing against each other for roles. The two sisters were students at the Diane Hill Hardin Young Actors Space school.
McKellar studied economics at Yale University from 1996 to 1999, graduating magna cum laude. While at Yale she was the editor of a magazine named Portia, and skiied for the university. She graduated from Harvard Law School (cum laude) in 2003.
When her sister won the part of Winnie Cooper in The Wonder Years, directors had strongly considered giving the part to Crystal. She in fact played Becky Slater, who became Winnie's romantic rival.
- Doubletree Hotel radio commercial (1984)
- Sunbelt Savings radio commercial (1984)
- Cabbage Patch Babies television commercial (1985)
- McDonalds television commercial (1985)
- The Wonder Years (1988–1993)
- Fatal Judgement (1988) television movie
- Judgement (1990) television movie, as Sabine Guitry
- 50 Simple Things Kids Can Do To Save The Earth (1992), television special.