Allmendinger was born in Santa Clara, California but raised in Los Gatos. Both suburbs of the San Francisco Bay area. Allmendinger was married to Canadian model and 2003 Miss Molson Indy Canada Lynne Kushnirenko in January 2007, but filed for divorce in 2012.
Allmendinger started his career in karts, winning two International Kart Federation Grand National championships.
He also raced in New Zealand in 2002 in the New Zealand Formula Ford Championship.
Allmendinger won the Barber Dodge Pro Series championship in 2002, and the Champ Car Atlantic Championship in 2003 with Carl Russo's RuSport team.
In 2004, Allmendinger and RuSport entered the Champ Car World Series, with Michel Jourdain Jr. joining Allmendinger. He scored a run of six top-six finishes towards the end of the season and won the Roshfrans Rookie-of-the-Year Award ahead of Justin Wilson. Wilson joined the team for 2005, with both drivers having been in regular contention for podium finishes.
On June 9, 2006, RuSport announced that Allmendinger would be replaced by 2002 CART champion Cristiano da Matta. Five days later, Forsythe Championship Racing announced Allmendinger as their new driver, replacing Mario Domínguez. In the interim, he became engaged to Lynne Kushnirenko, a Canadian model and chiropractor. In his first race with Forsythe, Allmendinger won the Grand Prix of Portland. Allmendinger became the first American to win a Champ Car World Series event since Ryan Hunter-Reay won at the Milwaukee Mile in 2004. This began a three-race winning streak that included wins at the Grand Prix of Cleveland and the Grand Prix of Toronto. This victory moved Allmendinger into second place in the CCWS championship standings, behind leader Sébastien Bourdais.
He chalked up his fourth and fifth wins of the season at Denver and Road America. He left Champ Car with one race left in the season to accept a lucrative offer from the NASCAR operation Team Red Bull.
Red Bull officially announced Allmendinger as part of its driver lineup October 25, 2006. He made his NASCAR debut in the Craftsman Truck Series driving for Bill Davis Racing at New Hampshire International Speedway on September 16, 2006, in the No. 24 Toyota Tundra. He crashed his primary truck in qualifying but started the race 32nd in a backup truck from teammate Bill Lester and finished on the lead lap in 13th. He also competed at the Talladega Superspeedway in October with a finish of fifth. In his third career truck series start, he qualified in second place at Atlanta and led five laps before he wrecked and finished 34th.
Allmendinger attempted his first Nextel Cup Series event at Atlanta in October 2006 driving the No. 84 Red Bull Dodge Charger; however, due to qualifying being rained out, a lack of owner's points prevented Allmendinger from making the race. He also attempted to make the race at Texas Motor Speedway, failing to qualify again. Allmendinger and Brian Vickers were named Red Bull's drivers for 2007, with Allmendinger piloting the team's No. 84 Toyota Camry with Vickers in the team's No. 83.
Allmendinger failed to qualify for the 2007 Daytona 500 after a crash in the first Gatorade Duel race. He also failed to qualify for the next four races before making his first Cup start at the Food City 500 at Bristol. As the season progressed, Allmendinger made a handful of races, predominantly in the "Car of Tomorrow". To assist him in the transition to stock cars, he participated in selected Craftsman Truck Series races in a Toyota for the Darrell Waltrip Motorsports organization, and in the No. 42 Memorex/Chip Ganassi Dodge in the Busch Series.
Allmendinger again failed to qualify for the 2008 Daytona 500. After three failures to qualify in two attempts (qualifying for the second race of the season, Fontana, was rained out and set by 2007 owner's points), he was replaced by veteran driver Mike Skinner on a temporary basis. Allmendinger returned to the Cup Series at Talladega. On May 17, he won the Sprint Showdown at Lowe's Motor Speedway during NASCAR's annual All-Star weekend. The win qualified Allmendinger for the Sprint All-Star Race later that evening, where he finished 17th. Due to Allmendinger's 11th-place finish at Watkins Glen, for the first time in his career, he had a guaranteed starting spot for the next race. He had his best career finish at the time at Kansas Speedway coming in ninth, but was released from the team two days later. He was replaced for the rest of the season by Scott Speed and Skinner.
After signing a one-race contract with Michael Waltrip Racing, Allmendinger replaced fellow former open-wheel star Patrick Carpentier at Gillett Evernham Motorsports, scoring an average finish of 15.4th over five races in the team's No. 10 Valvoline Dodge. During the offseason, as part of the merger between GEM and Petty Enterprises, Allmendinger was to move to the No. 19 Best Buy Dodge replacing Elliott Sadler. However, Sadler was still under contract for the 2010 season and announced he would file suit against Allmendinger and the organization. In early January 2009, Richard Petty Motorsports announced a settlement where Sadler would remain in the No. 19 and Allmendinger would return to his 2008 team, which was renumbered 44. However, Valvoline did not commit to sponsoring the team full-time and they were forced to race without full-time sponsorship.
Allmendinger's team ended the 2008 season 36th in owner's points, meaning it did not have exemptions for the first five races of 2009. The retro-styled Valvoline 44 raced its way into the Daytona 500, and third in his Daytona 500 debut, also his personal best, and is the fourth best Daytona 500 debut behind Lee Petty in the 1959 Daytona 500, Scott Wimmer in the 2004 Daytona 500 and Trevor Bayne in the 2011 Daytona 500. Allmendinger was one of two go-or-go-home drivers (the other being Tony Stewart, but Stewart had the past champion's provisional available) to successfully attempt the first five races of the 2009 season.
At 1:27 am on October 29, 2009, Allmendinger was arrested for drunk driving by Mooresville, North Carolina police. He registered a .08 blood alcohol level according to a police report. NASCAR placed him on probation for the rest of the 2009 season. Allmendinger drove a Ford Fusion in the final three races of the season as part of RPM's transition from Dodge to Ford.
During the offseason, Allmendinger replaced Reed Sorenson in the team's famous No. 43 car. He collected two top-fives, eight top-10s and a pole position in 2010, and finished 19th in the final standings.
After finishing 11th in the 2011 Daytona 500, Allmendinger started the season 10th in points driving the No. 43 Ford sponsored by Best Buy. Allmendinger continued to have a career best year in 2011 with 10 top-10 finishes and an average finish of 16th. He finished the 2011 regular season contending for a wild card spot in the Chase for the Sprint Cup but came up just short. Three races prior to setting the field for the Chase, RPM put former Roush Fenway Racing crew chief Greg Erwin on the pit box starting at the 2011 Brickyard 400. Allmendinger and Erwin recorded six top-10 finishes after their pairing. Allmendinger finished the 2011 season a career best 15th in the points. Allmendinger also announced that he would have a partnership stake in Mike Shank's IndyCar team MSR Indy.
At the end of the 2011 season, Allmendinger left Richard Petty Motorsports when the driving spot for Penske Racing's No. 22 Shell/Pennzoil Dodge became open after the parting ways of Kurt Busch and Penske. Prior to his suspension, his best finish was a second at Martinsville Speedway.
On January 28 and 29, 2012, Allmendinger entered the Rolex 24 driving for Michael Shank Racing and drove the final segment of the 24-hour race to the victory for the team in the Daytona Prototype Division. His teammates were Justin Wilson, John Pew, and Oswaldo Negri. To do so, he had to fend off sports car racing veteran Allan McNish among others.
After failing a random drug test on July 7, 2012, Allmendinger was suspended from participation in the Coke Zero 400. Steve O'Donnell, NASCAR's senior vice president for racing operations, said that Allmendinger had up to 72 hours to request a B test sample. The next day, Penske said before the Honda Indy Toronto race that Allmendinger's B sample would be tested on Monday or Tuesday. Allmendinger requested a B sample test on July 9. On July 11, 2012, Allmendinger's camp said a stimulant caused the positive drug test. The B sample test had not yet been scheduled at that time.
On July 24, it was announced by NASCAR that Allmendinger was suspended indefinitely after the B sample tested positive for a banned stimulant, which was revealed to be amphetamines. He chose to participate in the Road to Recovery program. On August 1, he was released from his contract by Penske Racing. Allmendinger was replaced in the No. 22 by Sam Hornish, Jr.; he later stated that the cause of the positive test was Adderall that he had unknowingly taken, being told it was an "energy pill". Allmendinger was reinstated by NASCAR on September 18 after completing the Road to Recovery program.
In October 2012, Allmendinger returned to NASCAR at Charlotte Motor Speedway, driving for Phoenix Racing, substituting for Phoenix's intended driver Regan Smith, who had been hired by Hendrick Motorsports to drive the No. 88 for Charlotte and Kansas while Dale Earnhardt, Jr. was sidelined with a concussion.
It was reported in late 2012 that Allmendinger could move to the IndyCar Series with Michael Shank Racing, which he co-owned, for the 2013 season. This failed to materialize; instead, Allmendinger signed with Phoenix Racing to run a limited schedule in the Sprint Cup Series in the No. 51 Chevrolet; in addition, he participated in IndyCar's winter testing with Team Penske, and in March it was announced that Allmendinger would compete in the IndyCar series for the team in at least two races, at Barber Motorsports Park and at the 2013 Indianapolis 500. This was extended to include the Long Beach Grand Prix and the Chevy Indy Dual in Detroit.
On June 1, 2013, Roger Penske announced that Allmendinger would drive for his team at Road America and the Mid-Ohio Sports Car Course. In his first race of the season, the Johnsonville Sausage 200 on June 22, he won after winning the pole position and leading 29 laps, which was the most of any driver. At Mid-Ohio in August, Allmendinger dominated the race, saving fuel through a green-white-checkered finish to win and sweep the year's Nationwide Series road-course races for Penske Racing.
Sprint Cup Series
Allmendinger drove part-time for Phoenix Racing in the No. 51 Chevrolet in the Sprint Cup Series. He had several strong finishes in the first part of the season: 11th at Phoenix, 13th at Bristol, 16th at Fontana, and 14th at Richmond. His first finish worse than 20th was at Pocono, where he finished 33rd. Afterwards, Allmendinger moved to the No. 47 Toyota at JTG Daugherty Racing in place of Bobby Labonte for two races, with a 19th-place finish at Michigan and 22nd-place finish at Kentucky. At Daytona, Allmendinger returned to the No. 51 and was running near the top-ten for almost the entire night until lap 148, when he was part of a wreck in the tri-oval with Denny Hamlin, Jeff Gordon, Matt Kenseth, and David Reutimann, reducing Allmendinger to a 35th-place finish.
Returning to the No. 47 at Watkins Glen, Allmendinger had another breakout run, qualifying fourth, and running in the top-fifteen for the most of the day, finishing in 10th place.
On August 29, 2013, the Sporting News reported that Allmendinger would be the full-time driver for JTG Daugherty Racing in 2014. This was confirmed a month later with Allmendinger signing a multi-year deal with the team.
Sprint Cup Series
Allmendinger had a few poor opening races, but did well at Fontana, recovering from a late speeding penalty to finish eighth, as well as having top-tens at Richmond and Talladega.
At Sonoma, Allmendinger qualified on the front row with Jamie McMurray. Allmendinger led the most laps and was in position to possibly earn his first Sprint Cup victory when he was wrecked by Dale Earnhardt Jr. with 35 laps to go. Allmendinger was visibly upset with Earnhardt Jr. in post-race ceremonies.
At Watkins Glen, Allmendinger took the lead with less than 30 laps to go after a side-by-side battle with former teammate Marcos Ambrose and kept the lead until the checkered flag, earning his first Sprint Cup Series win after 213 Sprint Cup starts, leading 30 laps in the process. "My gosh, I can't believe we won a NASCAR Sprint Cup race. This whole 47 team... all the sponsors, my first Cup victory. I love these guys. I just wanted it so bad for them. For this team. They worked so hard. I wasn't going to let Marcos take that from me. I hope the fans loved that race and at home. Because it was fun in the race car." Ambrose commented on Allmendinger's win: "First of all, congratulations to A. J. and the 47 team. They deserved that win. I left nothing on the table. I tried to rattle his cage and couldn't shake him. We raced fair and square to the end there. It was a tough couple laps but it was fair. We were both giving it to each other pretty hard. No harm, no foul. We just came up a little short." Before the race, Allmendinger had announced that Kimberly-Clark Corporation (consisting of Scott, Viva, Kleenex, Cottonelle and Huggies brands) extended its sponsorship agreement with the team through 2017.
On January 4, 2014, it was announced that Allmendinger would return to Michael Shank Racing for the 24 Hours of Daytona. Co-driving with John Pew, Oswaldo Negri, Jr. and Justin Wilson, the team finished 47th overall, 12th in the Prototype class.
Though NASCAR expanded the grid of the Sprint Unlimited to include all the Chase drivers, Allmendinger declined the invitation saying he wanted to save his equipment for the points races such as the Daytona 500. Instead, he joined the Motor Racing Network's radio broadcast of the race, working in the booth alongside Joe Moore and Jeff Striegle.
Allmendinger got his season off to a good start, leading a few laps during the Daytona 500. The next week at Atlanta, he finished seventh and sixth one week later at Las Vegas. On May 9, 2015, Allmendinger announced plans to sign a five-year contract extension with JTG Daugherty, allowing him to remain with the team through the 2020 season.
Allmendinger nearly pulled off an upset at the 2016 STP 500 at Martinsville. He charged hard from 13th place with less than 20 laps to go, to finish second to Kyle Busch by 1.547 seconds. It tied 2012 as his career-best finish at Martinsville. Allmendinger ended the season on a high note earning four top tens on the final 6 races and finishing 19th in the standings.
On June 8, 2016, Richard Childress Racing announced plans to add Allmendinger to their Xfinity Series driver lineup, hiring him to drive the No. 2 Chevrolet Camaro at Mid-Ohio; however, Allmendinger had a schedule conflict and Sam Hornish Jr. drove the race instead.
Allmendinger started the 2017 season at Daytona by finishing in third place, equalling his best career finish in the event. At Talladega, Allmendinger was caught in a multi-car accident while running third with 19 laps to go. Allmendinger got loose drafting Chase Elliott, causing Allmendinger's car to spin out and eventually flip over. At Watkins Glen, Allmendinger ran well enough to be the highest-finishing Chevrolet in the race, placing ninth.
(key) (Bold – Pole position awarded by qualifying time. Italics – Pole position earned by points standings or practice time. * – Most laps led.)
* Season still in progress.
1 Ineligible for series championship points.