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.
Two late June events provided a great start on the 2020 class and a look at some new 2019 talent.
ESPN Recruiting Insider Jeff Borzello discusses how Terrance Ferguson's decision to play basketball internationally will impact his positioning in the 2017 draft, as well as the Wildcats' upcoming season.
Miles Bridges shows the world he can touch the top of the backboard (with ease) in a test of his leaping ability.