Johnson worked for Fox for seven seasons, including four years at Georgia. He previously coached under Fox at Nevada.
Fox says Johnson "is like family to me" and says his contributions will be missed.
Georgia finished 15-17 overall and 9-9 in the Southeastern Conference last season. Fox is 65-63 in four seasons at Georgia.
Johnson played for coach Tubby Smith at Tulsa before Smith became Georgia's coach. He earned his law degree from Tulsa in 1999.
Texas Christian coach Trent Johnson preceded Fox as Nevada's coach from 2000-04. TCU has not announced Kwanza Johnson's hiring.
Copyright 2013 The Associated Press.