Rayshawn Simmons made a bucket to pull Central Michigan within two, 17-15, midway through the first half but back-to-back layups by Johnson sparked a 19-4 Western Kentucky run to close the half.
The Hilltoppers (3-2) led by as many as 36 in the second half.
Central Michigan (2-2) made just 8 of 34 (23.5 percent) field goals, including 2 of 15 from 3-point range, in the first half. The Chippewas didn't score after a dunk by DaRohn Scott with 5:59 left before the break.
Braylon Rayson scored 13, on 5-of-20 shooting, and Austin Stewart added 10 points for Central Michigan, which was without injured guard Chris Fowler - the program's all-time assist leader.
Western Kentucky recorded 21 assists on 31 field goals while shooting 53 percent overall.