Ehrenreich was born in Los Angeles, California, the only child of Sari (née Newmann), an interior designer, and Mark Ehrenreich, an accountant. He is named after the director Phil Alden Robinson, a family friend. His stepfather, Harry Aronowitz, is an orthodontist. Ehrenreich is Jewish; his ancestors were Jewish emigrants from Austria, Hungary, Russia, and Poland, and he was raised in Reconstructionist Judaism.
Ehrenreich began acting at the Palisades Elementary School in Pacific Palisades, Los Angeles, and continued doing so at Crossroads School in Santa Monica, California. After graduating from high school, he studied acting at New York University at the Stella Adler Studio of Acting, but never finished his degree. In 2009, he co-founded "The Collectin" with his friend Zoë Worth, a group of New York City actors, writers, producers and directors who collaborate on films and theater performances.