St. Mary's Maeghan O'Doherty's 30-yard shot with three minutes left sailed over keeper Laura Fawley, lifting the unranked Saints (7-4-1, 6-2-1) into a 3-3 Catholic League tie with top-ranked McDonogh (13-0-2).
O'Doherty's goal offset a hat trick by Eagles striker Laurie Schwoy and ended McDonogh's 10-game winning streak since an early-season 2-2 tie with No. 10 Mercy.
The Saints, who rode a four-game winning streak into the game, trailed 1-0 early and 2-1 at halftime before Kristen Buckley and Erin Hon tied the game each time.
"I think we just have a good team that comes out and plays hard," said St. Mary's coach Jerry Tobin.
McDonogh, ranked No. 3 by USA Today, is 32-0-2 since losing to St. Mary's in November 1992.
"The entire league gets sky-high to play us, and if we can't match their intensity, we can get taken on any given day," said Maurice Boylan, McDonogh's co-coach. "Fortunately, we didn't lose, so this could be the best thing to happen to us."