McCarthy announced his decision on Twitter on Tuesday night.
The 6-foot-5 McCarthy was a highly touted recruit by the Bruins three years ago, but he struggled with injuries and weight issues in three seasons at the school.
McCarthy had three sacks and 21 tackles last season as a reserve behind Eddie Vanderdoes and Kenny Clark. As a sophomore in 2013, he started eight games and was named the Bruins' most improved defensive player.
Quarterback Brett Hundley is the only other UCLA player who has headed to the NFL early this year.