Jayvon Graves added 18 points and six rebounds. Jeenathan Williams had 10 points while grabbing eight rebounds and Davonta Jordan chipped in eight points to go with seven assists and two steals.
Buffalo (2-1) took a 40-38 lead into the break. Harvard's Luka Sakota drained a 3 for a 46-45 lead with 17:52 left in the second half. Johnson answered with a trey and a dunk as the Bulls gradually pulled away for the win.
Bryce Aiken had 18 points, Justin Bassey added 12 and Chris Lewis chipped in 11 for Harvard (3-2).
Buffalo will play UConn Thursday and Harvard will face Holy Cross Friday.
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