Raised in Riverton, Illinois, Allgaier began racing at age five in quarter midgets, winning more than one hundred races and five championships before graduating to larger machinery. He entered stock car racing at thirteen in the UMP Late Model Series and made his ARCA Re/Max Series debut at sixteen at the Illinois State Fairgrounds Racetrack, driving for Ken Schrader.
Over the next several years he built steadily in ARCA, winning his first race at the Illinois State Fairgrounds in 2006 — becoming the first Springfield-area resident to win at the venue in its 72-year championship racing history. In 2008 he dominated the ARCA season, winning six races including the final three consecutive events to claim the series championship by a narrow margin, ending Frank Kimmel's eight-year streak of winning the title. He also made his NASCAR Craftsman Truck Series debut during this period, making four starts for MB Motorsports.
In May 2008, Allgaier signed with Penske Racing to drive four Nationwide Series races in the No. 12 Dodge. He was retained for a full 2009 campaign, earning the series Rookie of the Year award with Penske Championship Racing under Verizon sponsorship. He won his first career NASCAR pole at Memphis Motorsports Park that season.
Allgaier recorded his first Nationwide Series race win at Bristol Motor Speedway on March 20, 2010, and improved to fourth in the standings — the highest-finishing series regular that year, with the top three all being full-time Cup drivers. After moving to Turner Motorsports in 2011, he won the inaugural STP 300 at Chicagoland Speedway after Carl Edwards ran out of fuel on the final lap. In 2012, driving for Turner Scott Motorsports, he passed Jacques Villeneuve on the final lap to win the NAPA Auto Parts 200 at Circuit Gilles Villeneuve in Montreal.
Allgaier made his Sprint Cup Series debut in late 2013 at Chicagoland Speedway for Phoenix Racing. He then competed full-time in the Cup Series in 2014 with HScott Motorsports in the No. 51 Chevrolet, finishing 29th in the standings and third in the Rookie of the Year standings behind Austin Dillon and Kyle Larson. He departed HScott after the 2015 season and returned to the Xfinity Series with JR Motorsports.
After forming a connection with team owner Dale Earnhardt Jr. at Darlington Raceway, Allgaier joined JR Motorsports for the 2016 Xfinity season in the No. 7 Chevrolet. He went winless in 2016 but finished third in the championship behind Elliott Sadler and Daniel Suárez.
In 2017 Allgaier returned to winning, taking the DC Solar 200 at Phoenix and a second victory at Chicagoland — the first time in his career he had won multiple races in a single season. JR Motorsports achieved a 1-2-3 points sweep with William Byron champion, Sadler second, and Allgaier third.
The 2018 season saw five wins, including dominant performances at Iowa, Mid-Ohio, Road America, and Indianapolis Motor Speedway, where he led a race-high 41 laps to take victory. He won the 2018 Regular Season Championship but was eliminated in the Round of 8 at ISM Raceway.
Allgaier consistently made the Championship 4 in subsequent seasons. In 2023, he won at Charlotte, beat Sheldon Creed by 0.005 seconds in double overtime at the Daytona night race, and won at Bristol and Martinsville in the playoffs to reach the finale, finishing second in the championship. The 2024 season brought the championship he had long pursued: despite going winless in the playoffs, Allgaier remained consistent enough to make the Championship 4 on points and claimed the Xfinity Series title, with December 14 subsequently recognized as Justin Allgaier Day in Springfield, Illinois.
Throughout his Xfinity tenure, Allgaier made periodic Cup Series appearances as a substitute driver. In 2020 he started the Coca-Cola 600 in place of Kyle Larson after weather delayed the Indianapolis 500. In 2025, JR Motorsports made its Cup Series debut at the Daytona 500 with Allgaier driving the No. 40 Chevrolet, sponsored by Traveller Whiskey — he finished ninth, JRM's first Cup top-ten. Allgaier also made starts in the Porsche GT3 Cup Brasil series from 2016 onward as co-driver for Miguel Paludo in endurance rounds, accumulating several starts at Goiânia and Interlagos.
Allgaier attended Lincoln Land Community College, majoring in engineering. He is married to Ashley and has two daughters, Harper and Willow. He is nicknamed Little Gator — a play on his surname, with his father Mike having been dubbed Gator. Following his 2024 championship, December 9, 2025, was proclaimed Justin Allgaier Day across the entire state of Illinois.
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