Purdue defeated the Illini for the eighth straight time in what was the Big Ten opener for both schools.
Johnson scored 25 points with nine rebounds to give the Boilermakers their third straight win. They improved to 7-6 and dropped Illinois to 13-2.
D.J. Byrd added 15 points for Purdue and Jacob Lawson had 10 along with seven rebounds.
Illinois had a 47-44 lead when the Boilermakers embarked on their critical surge. Purdue limited Illinois to 35 percent shooting.
Brandon Paul scored 15 points for Illinois. After winning their first 12 games of the season, the Illini have dropped two out of three.