Gruen was born in New York City. He began photographing rock stars with Bob Dylan and served as John Lennon’s personal photographer during his time in New York City. Gruen is best known for his photograph of Lennon wearing a New York City T-shirt. Other notable celebrities and rock bands photographed by Gruen include the New York Dolls, The Clash, Ramones, Patti Smith Group, Blondie, Led Zeppelin, The Who, David Bowie, Tina Turner, Elton John, Aerosmith, Kiss, Alice Cooper and Green Day.