Hornaday was driving toward the win when Johnny Benson lost a tire and hit the wall to bring out a caution with 11 laps to go. It set up a restart with five to go, and Hornaday got a decent jump on Allmendinger at the line.
Allmendinger was closing when Eric Darnell crashed to bring out the seventh caution. It sent the race into overtime for a two-lap sprint to the finish.
Hornaday was the leader, with Allmendinger in second and series champion Todd Bodine in third. Hornaday again got a good start, and Allmendinger briefly closed in on his bumper. But he couldn't get close enough to attempt a pass, Hornaday pulled out and Allmendinger never challenged over the final lap.
"I thought he had us, but I ran him down low and blocked him," Hornaday said. "Made sure I took some air off his nose. Man, I am so excited."
It was Hornaday's 30th career truck victory, but the first at LMS. Hornaday, driving for Kevin Harvick Inc., also scored the first win of the season for Chevrolet after Toyotas won four of the first five events.
Allmendinger, the former Champ Car star, had a career-best second-place finish.
"Ron was just a touch better," Allmendinger said. "If he made a mistake, I was going to be all over him."
But after crashing his truck during an afternoon practice session, Allmendinger was thrilled to even compete. In the midst of a trying first season in NASCAR, Allmendinger has struggled to make Nextel Cup races - he's been in only four of 11 - and hasn't contended for a victory since the end of his open-wheel career.
"It's obviously an adjustment," Allmendinger said. "At the end of the Champ Car season last October, that really was the last time I was in contention. But I knew coming into this that it was going to be a challenge. You've got to reset your goals.
"Making Cup races right now, that's a victory to me. I was going to be pleased with a top 10, so second, it feels great."
Bodine was third, followed by Mark Martin, Ted Musgrave, Mike Bliss, Matt Crafton, Mike Skinner, T.J. Bell and Rick Crawford.