With Harrison hitting 5 of 8 shots, the Red Storm (5-1) opened a 35-24 lead on the Purple Eagles (1-4)
Niagara rallied in the second half to close within 46-45 on a Karonn Davis 3-pointer with 11:44 to play but Phil Greene IV had consecutive jumpers after a Rysheed Jordan free throw to put St. John's back up 51-45.
When the Purple Eagles got it to 53-30 on a Wesley Myers 3, the Red Storm went on a 12-0 run, started by a Sir'Dominic Pointer bucket. He had six in the run with two dunks and Jordan's 3 made it 65-50 with 4:49 remaining.
Jamal Branch had 12 for St. John's.