Slater was born Christian Michael Leonard Hawkins in New York, New York, the son of Mary Jo Slater, a casting executive, and Michael Hawkins, an actor who was also known as Michael Gainsborough. Slater has a maternal half-brother, Ryan, who is also an actor. Slater attended the Dalton School, the Professional Children's School and the Fiorello H. LaGuardia High School of Music & Art and Performing Arts.

He has been erroneously thought to be related to Helen Slater. In addition to their shared professional surname (his birth name is Hawkins), the myth is further fueled by their portrayal of sister and brother in The Legend of Billie Jean. Additionally, the DVD of Supergirl, featuring Helen Slater, included a critic's review which wrongly stated she is his sister.