The sixth seeded Mustangs held the lead most of the game over the third seeded Pirates.
Monte Vista led 7-6 at half. They scored two third quarter touchdowns to widen their margin to 21-6.
However, San Leandro crossed the goal line in the fourth quarter to cut the lead to 21-12 before Monte Vista put the game away with a touchdown with less than a minute to play.
Monte Vista is now 8-4. They will meet their cross-town rival, San Ramon Valley High, next weekend. The Mustangs defeated the Wolves, 34-33, in the final regular season game two weeks ago.
San Leandro finishes the season with a 6-5 record. The Pirates began the season by losing three of their first four games.
However, they reeled off five straight wins before losing to Bishop O'Dowd, 48-26, in a league championship battle two weeks ago.
San Leandro was given a number three seed in the Division I tournament and had a bye last week before matching up with Monte Vista.