The Ducks are winless in their first three games of the preseason.
The Sharks finished the night outshooting Anaheim 49-14.
Ryan Getzlaf scored a power-play goal for Anaheim.
The Sharks are 2-0 in the preseason.
The Ducks drew an unusual delay of game penalty at the start of the second period when they arrived late to the ice from the locker room and weren't prepared to start the period.
Andrew Murray's power-play goal at 18:15 of the third period made the score 5-1.
Mitchell's goal came on a power play at 17:25 of the second period, giving San Jose a 4-1 lead.
Anaheim made it 3-1 when Getzlaf scored 10 seconds into a power play at 14:56 of the second.
San Jose jumped out to a 2-0 lead late in the first period when Irwin and Boyle scored.
Irwin's score came on a power play at 15:21. The rookie defenseman was playing in his first NHL preseason game.
Boyle took a pass from Thornton and was quick to get set up and score with 1.4 seconds left in the period.
Notes: Sharks goalie Antti Niemi has yet to skate with the team after having a cyst removed last week. ... Last season's backup goalie, Antero Niittymaki, will be out 12 weeks after undergoing hip surgery earlier in the week.