Harrison, who leads the team in scoring at 17.7 points per game, did a lot more than shoot for the Red Storm (13-9, 3-6 Big East) as he finished with six rebounds, three assists, two steals and two of St. John's 12 blocked shots.
The Red Storm had one of their better defensive games of the season and the blocks had a lot to do with that.
St. John's came in leading the nation in blocks at 8.3 per game and Chris Obekpa, who had seven Saturday, came in seventh in the country at 3.3 per game.
Derrick Wilson had 14 points for Marquette (12-10, 4-5), which continued its streak of win one, lose one for the 12th consecutive game. The loss dropped the Golden Eagles, the preseason favorite to win the conference, into a tie for fifth place with Seton Hall.
The Golden Eagles, who came in as the lowest scoring team in the 10-team conference (71.0), struggled all game with their shooting. Marquette shot 34.8 percent for the game (23 of 66), including 3 of 14 (21.4 percent) from 3-point range, another statistical category it is last in in the Big East at 29.7 percent.
Marquette, which opened the game on a 6-0 run, had won 13 of the last 14 meetings between the schools.
St. John's held Marquette to three field goals over the last 12:45 of the first half and was able to take a 33-20 lead.
The Red Storm pulled away to the big lead with a 15-2 run that Harrison started with a 3-pointer and a four-point play. He was fouled by Deonte Burton as he hit his sixth 3 of the game. Harrison jumped up from the court and screamed toward the crowd. His free throw made it 51-33. The run ended when freshman Rysheed Jordan dunked on a breakaway to make it 59-35 with 11:24 to go.