Morgan Park won despite losing top-ranked junior guard Ayo Dosunmu to a foot injury.
Morgan Park trailed by as many as 11 points in the fourth quarter ahead of changing the game with their defensive pressure.
Freshman guard Nimari Burnett led the way with 24 points. Burnett knocked down 6-10 shots from distance including a trio of big triples in overtime.
Fenwick was led by freshman guard D.J. Steward with 26 points.
The state title was the third for Morgan Park head coach Nick Irvin who also won titles in 2013 and 2014.
Morgan Park defeated Fenwick 69-67 in overtime to win the Class 3A state title on Saturday. Morgan Park has won the 3A title in three of the past five years.