Gray was born in Artesia, New Mexico, into a motorsport family. His grandfather, Johnny Gray, is a Funny Car and Pro Stock champion and later became co-owner of Tricon Garage; his father, Shane Gray, is a professional drag racer and NHRA national event champion. The family relocated to Mooresville, North Carolina in 2010 to support Shane Gray's professional NHRA career. Tanner's younger brother, Taylor Gray, also races professionally. As a third-generation NHRA Gray competitor, Tanner came to the sport with considerable family knowledge and experience.
Gray began racing in the NHRA Pro Stock class in 2017 alongside his father Shane Gray. On April 2, 2017 — thirteen days before his eighteenth birthday — he won the 18th Annual Denso Spark Plug Nationals at The Strip at Las Vegas Motor Speedway, defeating Bo Butner in the final round. At 17 years, 11 months, and 18 days old, he became the youngest driver to win an NHRA national event in history, breaking a 40-year record previously held by Jeb Allen, who was 18 years, 1 month, and 8 days when he set the mark. Gray also won the 2017 Auto Club Road to the Future Award, given to the top rookie across all pro categories in the NHRA.
In 2018, Gray won eight NHRA Pro Stock national events and claimed the Pro Stock World Championship, making him the youngest NHRA season champion ever. The title confirmed his debut season's promise and capped a two-year stretch in which he set records that had stood for decades.
Following his NHRA success, Gray transitioned to stock car racing. He competed in the NASCAR K&N Pro Series East beginning in 2019 for DGR-Crosley, earning his first win at South Boston Speedway in early May of that year.
Gray made his NASCAR Truck Series debut in October 2019 at Martinsville Speedway with DGR-Crosley. In 2020 he ran a full rookie season in the Truck Series with the team, recording eight top tens and four top fives in a solid debut. He continued in the series through subsequent seasons, improving steadily. In 2023, running under the newly renamed Tricon Garage banner after the team aligned with Toyota Racing Development, he started the season with a career-best second-place finish at Daytona and scored his first career pole at Charlotte Motor Speedway with a lap of 180.385 mph (290.302 km/h). His contract with Tricon Garage was renewed for 2025.
Gray also ran limited ARCA Menards Series events, including appearances in 2024 for Joe Gibbs Racing at tracks 1.5 miles and longer.
Gray has raced a variety of car types throughout his career, including late models, modifieds, midget cars, and stock cars, competing on oval dirt tracks in and around Mooresville. He raced in the Mini Outlaw Karts category alongside Kyle Larson and Rico Abreu at Millbridge Speedway in Salisbury, North Carolina.
Gray's NHRA records — youngest national event winner and youngest season champion — are products of a prodigious early career that drew on three generations of family motorsport expertise. His subsequent transition to NASCAR demonstrated versatility across disciplines, and he has established himself as a consistent Truck Series competitor while his record-setting drag racing achievements remain intact.