The No. 4-seed Terps will face the No. 5-seed Iowa Hawkeyes on Wednesday for a spot in the semifinals.
The goal lifted the Terps to a weekend sweep of the Scarlet Knights.
All four Maryland goals resulted from penalty corners.
Bibi Donraat scored the winner in the 58th minute.
Wildcats forward Bente Baekers slotted past Noelle Frost late in the fourth quarter to secure the victory.
Donnelly has taken more shots than anyone else in the Big Ten through two games.
The Terps failed to find the back of the net on nine penalty corner attempts against Northwestern.
“She did her time as a redshirt. She’s now got the groove, and she knows what it takes.”
Goalkeeper Noelle Frost posted 11 saves, but the Terps were unable to stymie the Cavaliers' attack in overtime.