Donovan scored on a header and with a right-foot shot from 12 yards that Mainz's keeper Christian Wetklo failed to handle and let slip over the goal line.
Donovan's first goal VIDEO.
Second goal VIDEO.
Donovan also had a goal called back for offside. Miroslav Klose, Luca Toni and Bastian Schweinsteiger got Bayern's other goals. The game drew 16,000 fans in Mainz, which is tied for first place in the German Second Division with Kaiserslautern, against which Donovan scored in a 2-0 win on Monday.
In five friendly games, Donovan, who is on loan until the MLS season starts in mid-March, leads Bayern in scoring ahead of Klose (3 goals), Toni (2), Schweinsteiger (2) and Tim Borowski (2).
Donovan scored his first against Mainz 10 minutes after entering the game. He headed home a cross from Italian Massimo Oddo. Against Kaiserslautern, Donovan scored 12 minutes after subbing in, also on a header from a Oddo cross.
The goals off the bench may increase Bayern's interest in buying Donovan from MLS as Coach Juergen Klinsmann is looking for an effective striker to back up Toni and Klose. The German TV broadcast showed Bayern general manager Uli Hoeness nodding approvingly after Donovan's strike. Whether Donovan gets his wish of a transfer depends greatly on whether Hoeness is willing to pay what MLS would ask - which could be up to $10 million.
In the 83rd minute, Donovan was called offside after he buried a rebound off a Toni shot. In the 90th minute, Donovan met a low cross from Hamit Altintop and rifled it at Wetklo, who could only deflect it over his goal line.
Donovan was asked before the game by Germany's Abendzeitung newspaper on when a decision would be made on a long-term contract with Bayern.
"There's no date," Donovan said. "If I go back to the Galaxy, we have our first game March 21, so I'd have to return at least one week earlier. But that would be sad. I want to make it here. It is my dream. I want stay here a few years."