Mayer, who entered the final race of the Round of 12 last in the standings, ran down Cole Custer and passed him for the lead with four of 67 laps remaining and held on to clear him by 0.909 seconds at the checkered flag.
The victory automatically advances Mayer into the semifinal round of the playoffs – the only way he could continue his quest for his first series championship.
It’s Mayer’s third win of the season – all coming on road courses (Road America and Watkins Glen were the others). He led a race-high 50 laps – the most in his career.
Mayer was among the lead-lap cars that pit under a late caution for new tires. He lined up sixth on a restart with eighth laps remaining and had moved up to second on the final restart with five to go.
“I knew we had time. Our car was so fast,” Mayer said. “It really felt unbeatable. I mean, what a day. The good Lord, he works in mysterious ways. He put us down, but he also taught me a lesson and brought me back up. We kicked their tails today. It just feels so great.
“Winning solves everything. That’s the name of the game. Getting another shot at it, I think we can make something out of it.”
Josh Berry finished third in the race, Rilery Herbst fourth and Parker Kligerman rounded out the top five.
Completing the top-10 were Kaz Grala, Daniel Hemric, John Hunter Nemechek, Austin Hill and Sheldon Creed.
Joining Mayer and Custer in the semifinal round of the playoffs are Nemechek, Justin Allgaier, Hill, Chandler Smith, Sammy Smith and Creed.
Stage 1
With several lead-lap cars pitting with three of 20 laps to go, Allgaier cruised to a 7.4476-second Stage 1 win over Hemric. Creed ended up third, Kligerman was fourth and Hill rounded out the top five.
Stage 2
Nemechek claimed the Stage 2 win under caution when Conor Daly spun and hit the wall in oval Turn 4 with two of 20 laps remaining. Hemric was second, Kligerman third, Creed fourth and Alex Labbe was fifth.
On Lap 33, Berry spun around in the areas of Turns 4 and 5. He was able to continue on but dropped from second to 13th in the running order.
Stage 3
During the break between Stages 2 and 3, most of the lead-lap cars pit with Hemric first off pit road. Mayer stayed out to inherit the lead and led the way on the restart with 24 laps remaining.
With 15 laps remaining, Mayer had built a more than 1-second lead over Custer as Allgaier ran third more than four seconds behind the leader.
Debris on the track near the exit of the frontstretch chicane forced NASCAR to display a caution on lap 56. Several teams elected to pit for new tires with Custer first off pit road.
Jeb Burton – who needed a win to advance in the playoffs – remained on the track and took over the lead. He was followed by Allgaier when the race returned to green with eight laps to go. Mayer lined up sixth.
As the field entered Turn 1, Burton got loose and slid into Allgaier while Custer drove by and into the lead. Burton, Allgaier and Blaine Perkins all slammed into the wall to put the race back under caution.
Custer led Mayer and Berry when the race returned to green with five laps remaining in the race.
With four laps to go, Mayer got around Custer in Turns 3 and 4 and reclaimed the lead.