The Illini (17-12, 6-10 Big Ten) hadn’t won in February and had lost nine of 10 since defeating Ohio State in early January.
Brandon Paul added 17 points and D.J. Richardson scored 12 for Illinois.
Leonard had 15 points by halftime.
Matt Gatens powered the Hawkeyes (15-14, 7-9) with 22 points and 5 of 6 shooting from 3-point range. But Gatens was held in check by Paul in the second half, scoring just five points.
Illinois led 34-23 at the break and weathered a Hawkeye run that closed the gap to 38-34 early in the second half.
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