Donovan, who is retiring at the end of the season, struck a hat-trick as the Galaxy thrashed Real Salt Lake 5-0 in their second leg at the StubHub Center.
It sealed a 5-0 aggregate victory and sent the Galaxy into the Western Conference Finals, where they will face either the Seattle Sounders or FC Dallas.
Bruce Arena's men had failed to score in two matches heading into the second leg, but that all changed thanks mainly to Donovan.
He opened the scoring in the 11th minute, heading in a cross from AJ DeLaGarza.
Donovan turned provider nine minutes later as he squared a pass for Robbie Keane, who had worked his way back into an onside position, to tap in.
Keane and Donovan combined again in the 54th minute, with the Irishman putting the latter through before the former United States international rounded goalkeeper Nick Rimando to make it 3-0.
Marcelo Sarvas completed a fine team move in the 63rd minute before Donovan sealed his hat-trick, poking a wonderful Keane pass past Rimando.
The Revolution sealed a place in the Eastern Conference Finals, beating the Columbus Crew 3-1 for a 7-3 aggregate victory.
The Crew needed a huge turnaround to advance, and the chances of that happening took a blow when Lee Nguyen scored from close range for the Revolution just before half-time.
Jose Goncalves made it 2-0 on the night for New England with a tap in.
Ethan Finlay and Justin Meram saw red for the Crew late on, with Tony Tchani and Teal Bunbury trading goals in a contest that was over.
The Revolution will face the New York Red Bulls in the Eastern Conference Finals.
