Mosack, who only began racing at age 17, took control of Friday’s 100-lap race and lapped every car in the field except for second-place finisher Carson Kvapil, who was making his series debut.
Mosack, who has raced predominately Late Models and in the SCCA Trans Am Series until venturing into ARCA and NASCAR competition beginning in 2021, is amid his most expansive stock car schedule yet.
Friday’s race was the last of a six-race ARCA schedule this season with Joe Gibbs Racing.
His previous best finishes were a pair of seconds – at Daytona earlier this year and last season on the road course at Portland (Ore.) International Raceway.
“First, I just want to thank everyone at Toyota and Joe Gibbs Racing. We’ve worked really hard all year, had some bad luck.,” Mosack, 24, said. “I’ve made plenty of mistakes. I feel like we earned that one and we gave a few others away.
“Today, we just went to work to get the balance right. I was definitely holding my breath there at the end, but we got it done.”
Kvapil, a Late Model star and son of former NASCAR driver Travis Kvapil, finished 3.2 seconds behind Mosack.
Toni Breidinger ended up third, Lavar Scott was fourth and Jake Finch rounded out the top-five.
Several top contenders saw their chances at a win slip away, beginning with pole winner Jesse Love, who blew a tire, hit the wall and wrecked out of the race after 26 laps.
Love still leads the series standings with three races remaining this season behind nine victories.
Love’s Venturini Motorsports’s teammate, Dean Thompson, led 35 laps but developed problems late in the event and was forced to pit under green.