Mesa was stuffed on fourth down and had the first half run out to account for the only other times it did not find the end zone.
The hosts (2-1) scored on all five second-half possessions, including an 80-yard kickoff return by senior Mario Smith and a 20-yard interception return by junior Noah JeyaRajah.
The two aforementioned scores came within an 86-second span and gave the Mustangs a 49-36 advantage with 6:54 left in the game.
The two quick-strike touchdowns helped erase a pair of one-play touchdown drives in the first half by the Knights (1-1).
Costa Mesa went 65 yards on eight plays with the second-half kickoff, as Oronde Crenshaw capped the march with a one-yard scoring run. Jake Lux added one of his eight conversion kicks to draw the Mustangs within 29-28.
The ensuing Katella possession resulted in the only punt of the game and after taking over on its own 11, Mesa used eight plays to find paydirt, as Justin Smith bolted 65 yards for the go-ahead score with 3:39 left.
Katella answered with a 15-play, 78-yard procession that put the visitors up, 36-35.
But Mario Smith, who carried 23 times for 184 yards and now has 487 rushing yards in his three-game Mesa career, returned the ensuing kickoff to put the Mustangs ahead for good.
JeyaRajah snatched a would-be screen pass and went untouched into the end zone three plays into Katella's next possession.
And after the Knights rolled 76 yards on seven plays to pull to within 49-42 with 4:09 left, the Mustangs' Jake Comer recovered an onside kickoff at the Katella 48-yard line.
Mario Smith did most of the work on the ensuing possession, carrying four times to the 12. Then Crenshaw bulled 12 yards for the capping score with 2:29 left.
Katella, which outlasted the Mustangs last season for a 35-34 overtime triumph, drove to the Mesa eight-yard line before time expired.