Juan'ya Green scored 17 points, T.J. Cline had 15 and Ameen Tanksley added 12 points and a team-high nine rebounds for Niagara.
The Purple Eagles (1-1) led 8-5 before St. Francis' Ollie Jackson knocked down a 3-pointer to even the score 8-all.
Niagara then made six of its next eight shots to take a 21-9 lead with 8:48 left in the first half. Cline capped the run with back-to-back layups and Mason converted two free throws at the end of the period to give the Purple Eagles a 45-25 halftime lead.
St. Francis (0-2) cut the deficit to 12, 64-52, in the second half but Green scored five straight points and Mason sank a jumper to seal the win.
Stephon Whyatt led St. Francis with 23 points and Jackson added 15.