Mason's 3-pointer with 6:09 remaining in the first half put the Purple Eagles (8-8, 4-1 Metro Atlantic Athletic Conference) ahead to stay, 21-19, and Green followed with two 3-pointers and a 2-point jumper for a 10-point lead, 29-19.
The Peacocks (6-9, 1-4) scored the game's first seven points before Niagara began to right itself.
Mason scored 19 of his points in the second half, making 7 of 9 shots, to keep the Purple Eagles comfortably in front.
Ameen Tanksley had nine points, seven rebounds and five steals for Niagara, which shares the MAAC lead.
Chris Prescott's three-point play with 1:10 remaining gave the Peacocks their only made free throw of the game in five attempts.
Oklahoma State basketball player Tyrek Coger died after a 40-minute team workout on the football stadium stairs in hot weather, university officials said Friday.
Prospective NCAA championship host cities must identify local anti-discrimination laws in a questionnaire being sent out by the college sports governing body.
The Class of 2017 is full of talented big men, but Richards is getting noticed by more programs during the current evaluation period in Las Vegas.