Horovitz was born in South Orange, New Jersey, the son of Doris Keefe and playwright Israel Horovitz. He began his music career with a stint in the punk rock band, The Young and the Useless, who would often perform with Beastie Boys. When Beastie Boys guitarist John Berry quit, Horovitz was an obvious choice to replace him.

In addition to his work with the Beastie Boys, Horovitz also remixes numerous tracks for other artists under the alias 41 Small Stars.

Horovitz has also acted in several motion pictures and television shows. Some of his roles include Tim 'Chino' Doolan in Lost Angels (1989), Sam in Roadside Prophets (1993), and Repulski in Godspeed (2007).

Adam is married to Kathleen Hanna of riot grrrl punk band Bikini Kill. The two have been together since 1998, marrying in 2006.[citation needed] He was formerly married to Ione Skye.