Frantz Massenat led Drexel with 17 points and freshman Tavon Allen added 16 in his first collegiate start after scoring 15 off the bench in a 61-59 win over Penn last weekend.
Yet Dellavedova - who started for an Australian squad that reached the Olympic quarterfinals - made the biggest impression on Dragons coach Bruiser Flint.
"He's a tough kid to play against," Flint said. "One of the things you've got to worry about with him is that he's a good passer. You can't be throwing three guys at him because he's going to find the guy who's open.
"He got guys the ball when they needed it. When they needed buckets, he got them. He really makes the other kids on the team a lot better."
The Dragons began the second half by making four successive baskets to draw within 40-38 with 17:22 to play. Then Dellavedova and Brad Waldow each scored five points in a 10-3 surge that gave the Gaels a 50-41 lead with 14:33 left.
Drexel's offense collapsed late. The Dragons made only one basket in a span of 6:11, while scoring seven points on free throws, as St. Mary's led by as many as 15 points.
"I thought we played hard but I didn't think we played smart," Flint said. "A big part of the game, I didn't think we played tough enough. We didn't do our job in the heat of the game, being in certain places, doing certain things."
Drexel sophomore guard Damion Lee, who has been averaging 12.0 points per game, missed his second consecutive game with an injured back and neck.