Denny Hamlin led the way entering the second, two-lap overtime but Chase Briscoe ran off course and cut the track and was immediately assessed a penalty by NASCAR.
Briscoe, however, continued racing on the track and raced Hamlin hard for the lead, hit him in the right-rear corner and ended up turning him.
Allmendinger, running third at the time, inherited the lead and ended up holding off Ryan Blaney by 0.929 seconds to win Sunday’s Verizon 200, the Cup series’ first race on the 2.439-mile, 14-turn road course at Indianapolis Motor Speedway.
“I was so mad yesterday – I wanted to win so bad! This is unbelievable,” Allmendinger said. “We had to fight hard. Robin Miller – I saw him this morning – this one’s for you baby, and Bob Jenkins.
“I just won Indy … I just won Indy!”
Allmendinger currently competes fulltime for Kaulig Racing in the Xfinity Series but has been driving a partial Cup schedule for the organization as well.
The win is the second of Allmendinger’s Cup career, his first coming in 2014 at Watkins Glen (N.Y.) International with JTG Daugherty Racing.
“It was survival of the fittest. We probably had like an eighth-, 10th-place car, sped on pit road. I thought we were going to finish 12th to 15th, and then those restarts were just insane,” Allmendinger said.
“It’s great when you have a car owner that just says, Go get me trophies. He doesn’t care if that thing is torn up. Chevy, ECR horsepower, RCR, everybody that allows us to do that, all of our sponsors on the Xfinity side of it, we just won at Indy.
“What’s up? Thank you everybody for coming out. Let’s go!
Kyle Larson ended up third, Chase Elliott fourth and Matt DiBenedetto rounded out the top-five.
Completing the top-10 were Kurt Busch, Erik Jones, Justin Haley, Austin Cindric and Ryan Newman.
Hamlin ended up 23rd and Briscoe was scored 26th and one lap down after his penalty for cutting the course.
“To me just lack of situational awareness,” Hamlin said. “I’m not like furious. It just sucks. Turned our day upside down.”
Following the break between Stages 2 and 3, a handful of lead-lap cars hit pit road which allowed Larson to inherit the lead.
On the restart on Lap 40, Larson was followed by Elliott, Kyle Busch, William Byron and Cole Custer.
NASCAR displayed a caution on Lap 41 to allow track workers to repair the curbing in Turn 6, which appeared to have a piece of a car’s splitter lodged underneath it.
Just before the caution, Ricky Stenhouse Jr. and Aric Almirola made contact in the same area and both spun but were able to continue on.
The race returned to green on Lap 44 with Larson still in the lead.
On Lap 52, Kurt Busch got into Cindric and spent Cindric around but both were able to continue without a caution.
Briscoe pit and a handful of others hit pit road with 31 laps remaining to make the final stop for new tires and fuel to make it to the finish of the race.
Elliott, Christopher Bell, Blaney and Martin Truex Jr. – who also reported a loose wheel – made their stops on Lap 55.
With 28 laps to go, Larson made his top, which turned the lead over to Hamlin.
Hamlin went briefly off course in Turn 12 on Lap 58 but was able to continue and remained in the lead as Kurt Busch moved into second.
Larson finally cycled back around to the lead on Lap 64 as first Hamlin and then DiBenedetto made their final green-flag pit stops.
On Lap 73, NASCAR was forced to throw a caution as large piece of debris ended up in the racing groove in the area of Turns 5 and 6.
DiBenedetto had hit pit road just before the caution flew and most of the lead-lap cars elected to pit under the caution.
Hamlin stayed out, as did Kurt Busch, DiBenedetto and Briscoe and they led the field on the restart on Lap 77. Larson lined up fifth, the first car off pit road.
On Lap 78, Larson went over the curbing in Turn 6, which came apart. Byron was the next car over the curb and it tore his radiator out, which also dropped fluid over the track.
Joey Logano slammed into the tire barrier and Kyle Busch, Daniel Suarez, Christopher Bell and Kevin Harvick also were caught up in the incident. The race was immediately red-flagged.
After a nearly 20-minute red flag that include removal of the curbing in Turn 6 and replacement of a section of tire barrier, the race returned to caution.
The race went into a two-lap overtime. On the restart, Hamlin led the way followed by Larson, Briscoe, DiBenedetto and Blaney.
On the restart, Michael McDowell went over the remaining curbing in Turn 6, which launched him around and set off another multi-car wreck and a second red-flag. Larson caught a piece of that accident and was relegated back to seventh.
Hamlin led on the start of the second overtime, followed by Briscoe, Allmendinger, DiBenedetto and Blaney.
Tyler Reddick held off Austin Dillon to complete a sweep of wins in the first two stages.
Ryan Preece was third, Stenhouse fourth and DiBenedetto rounded out the top-five.
Following the break between Stages 1 and 2, those cars that did not pit before the stage break did so, which turned the lead over to Brad Keselowski, who stayed out.
On the restart on Lap 20, Keselowski was followed by Elliott, Byron, Larson and Briscoe, who lost four spots from a NASCAR penalty for not maintaining pace car speed under caution.
Elliott powered past Keselowski after the restart to grab the lead for the first time as Logano moved to second and Larson to third.
On Lap 24, Ross Chastain spun around in Turn 1 but was able to continue on without a caution.
On Lap 26, Keselowski spun off Turn 11 and backed his No. 2 Ford into the wall but was able to get to pit road and no caution was called.
With five laps remaining in the second stage, Elliott had opened a 1.2-second lead over Larson as Kyle Busch ran third and Custer fourth.
On Lap 33, several of the lead-lap cars elected to pit for new tires and fuel before the stage break. Truex, Briscoe, Byron and Blaney were among the first to pit.
Reddick elected not to pit and moved back into the lead with one lap to go in the stage. Allmendinger was penalized for speeding on pit road during his stop.
Reddick passed McDowell with one lap remaining and held on to take the Stage 1 victory, his second of the 2021 season.
Austin Dillon was third, Jones was fourth and Preece rounded out the top-five.
Byron started on the pole and led the first lap but Briscoe got around him on Lap 2 to move into the lead.
On Lap 4, Truex was forced to pit under green with a tire rub. He returned to the track in 39th.
With 10 laps remaining in the stage, Briscoe had built more than a half-second lead over Byron as Elliott ran third.
On Lap 9, Almirola was also forced to pit under green with a tire rub.
With five laps to go, Briscoe had collected some grass on his grille but still lead Byron by 0.603 seconds as Elliott remained in third.
Most of the lead-lap cars elected to pit before the stage break on Lap 13, which moved McDowell into the lead.