Jason Alexander

Greenspan was born in Newark, New Jersey to a Jewish family, the son of Ruth Minnie (née Simon), a nurse and health care administrator, and Alexander B. Greenspan, an accounting manager. Greenspan later borrowed his father's first name to create his stage name, Jason Alexander.

Alexander grew up in Maplewood and Livingston, New Jersey, and is a 1977 graduate of Livingston High School. Interested in magic from an early age, he initially hoped to be a magician, but while attending a magic camp was told that his hands were too small for card magic. He became interested in theater, eventually realizing, "Wait a minute—the whole thing's an illusion. Nothing up there is real" and that theater itself was "a magic trick". He then decided to pursue a theater career.

After high school, he studied theater at Boston University. He wanted to pursue classical acting, but a professor redirected him toward comedy after noticing his physique, remarking, "I know your heart and soul are Hamlet, but you will never play Hamlet." Alexander left Boston University without a degree after his third year to take a full-time acting job in New York City. The university awarded him an honorary degree in 1995.