He scored his fifth goal of the year with 4:08 left in regulation time. The centre took a shot from behind the net and had it bounce off Otters' goalie Ryan Ludzik's blocker before sliding across the goal line.
The Otters didn't waste any time in replying with a quick goal off the faceoff from defenceman Josh Kidd. Matt Caria then sealed the win for the Majors with an empty netter with less than a minute to go.
Majors coach Bud Stefanski said both Donati and Caria scoring is essential to the clubs' success.
"When Justin scores, we win. When Matt scores, we win. They're two guys we've been counting on from the start. Those two guys mean a lot."
Micheal Haley had two goals for St. Mike's (5-6-0-0). Jason Cassidy, Cassidy Preston and Kaspars Daugavins also scored for the Majors.
Ryan O'Marra, Nick Palmieri and Patrick Lee each scored for the Otters (2-7-0-2).
Wayne Savage made 28 of 35 saves in net for the Majors. Otters starter Jeff Dawson gave up 4 goals on 19 shots. Ludzik stopped 20 of 22 in relief.