Custer was born in Ladera Ranch, California, to Joe Custer, the team president of Haas Factory Team and the chief operating officer of Haas F1 Team. He began racing quarter midgets at the age of four. Custer attended Ladera Ranch Middle School and Tesoro High School.
In 2011, Custer won the USAC National Focus Young Guns Championship. The following year, he began racing late models, earning Rookie of the Year honors after winning ten races. In 2013, Custer joined the K&N Pro Series East, making his debut at Bristol Motor Speedway for Ken Schrader Racing. He won at Iowa Speedway, leading every lap and becoming the youngest race winner in K&N Pro Series history at fifteen years old. He also won at New Hampshire Motor Speedway, again starting from the pole.
In 2014, Custer won the Pro Series West opener at Phoenix International Raceway. He began racing in the NASCAR Camping World Truck Series in 2014 with Haas Racing Development, setting a track record at Gateway Motorsports Park and earning his first Truck Series win at New Hampshire Motor Speedway. In 2015, he won the Truck race at Gateway Motorsports Park, holding off Spencer Gallagher.
In 2017, Custer began driving full-time in the Xfinity Series for Stewart–Haas Racing, scoring his first win at Homestead–Miami Speedway. The following year, he won at Texas Motor Speedway, securing a spot in the Championship Four. The 2019 season saw Custer win at Fontana, Richmond, Pocono, Chicago, Kentucky, and Dover. He was declared the winner at Darlington after Denny Hamlin was disqualified. He finished second in the points standings to Tyler Reddick at Homestead.
At Canadian Tire Motorsport Park in 2016, Custer was involved in an on-track incident with John Hunter Nemechek. Nemechek bumped Custer, sending both drivers off-road, and Nemechek was subsequently tackled by Custer after being declared the winner. In 2019 at Kansas, Custer got into a fight with Tyler Reddick on pit road.
In 2022, Custer and crew chief Mike Shiplett were fined US$100,000 and Shiplett was indefinitely suspended after Custer intentionally slowed down on the last lap of the Charlotte Roval race to help teammate Chase Briscoe advance in the playoffs.
Custer continues to compete in the NASCAR Cup Series as of May 28, 2024, when Stewart–Haas Racing announced it would shut down its NASCAR operations at the end of the season. He will continue to drive the No. 41 for the reorganized Haas Factory Team in 2025.
Custer has also competed in the NASCAR O'Reilly Auto Parts Series, driving part-time for SS-Green Light Racing with BRK Racing. He returned to the Xfinity Series in 2021 and 2022, winning at Auto Club Speedway in 2022. He also made starts in the ARCA Racing Series and ARCA Menards Series West.
Custer was awarded the Sunoco Rookie of the Year honors in the NASCAR Cup Series in 2020. He is a 2023 NASCAR Xfinity Series champion.
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