Raised in Mooresville, North Carolina, Nemechek attended Davidson Day School and began racing go-karts at age five. He moved into stock car competition in 2010, entering the Allison Legacy Series with sponsorship from England Stove Works. In 2012, he was the subject of significant attention after winning the pole for an ASA Midwest Tour event at the Milwaukee Mile just after his fifteenth birthday. After competing in the 2012 Snowball Derby, he won the Allison Legacy Series championship, claiming fifteen of the season's eighteen races. He also won the 300-lap Snowball Derby in 2014.
Nemechek made his Truck Series debut in late 2013 with SWM-NEMCO Motorsports, the family operation co-owned by his father. He ran limited schedules in 2013 and 2014 before competing in most races during the 2015 season, when he won at Chicagoland on September 19 — sixteen years to the day his father had won his first Cup Series race. He also received NASCAR's Most Popular Driver Award that year.
He won his first full-time Truck season in 2016, taking a victory at Atlanta but also drawing attention at Canadian Tire Motorsports Park, where a contact incident with Cole Custer ended with Custer tackling Nemechek on track before Nemechek was declared the winner. In 2017, he won back-to-back at Gateway and Iowa and narrowly missed the Championship 4 after finishing second at Phoenix. He added a win at Martinsville in 2018 before moving to a part-time schedule.
After signing with Kyle Busch Motorsports for 2021, Nemechek had his most dominant Truck season, winning at Las Vegas, Richmond, Charlotte, Texas, and Pocono, clinching the regular season championship, and finishing third overall. In 2022 with KBM, he won at Darlington and Kansas and finished fifth in the standings. He drove two part-time races for Tricon Garage in 2023.
Nemechek joined Chip Ganassi Racing for the 2018 Xfinity season on a part-time basis, winning at Kansas. He moved to GMS Racing for a full 2019 schedule in the No. 23 Chevrolet and on November 9, 2019, he and his father Joe made history at Phoenix by becoming the first father-son duo to compete in all three of NASCAR's main series in a single weekend. Nemechek finished seventh in points.
He returned to a full-time Xfinity schedule with Joe Gibbs Racing in 2023, driving the No. 20 Toyota. He won six races during the regular season at Fontana, Martinsville, Atlanta, New Hampshire, Michigan, and Kansas, then added a playoff win at Texas. In 2024, sharing the No. 20 with Aric Almirola while running Cup duties, he won at Las Vegas and Nashville.
Nemechek filled in for Matt Tifft at Front Row Motorsports for the final three races of the 2019 season, then joined FRM full-time in 2020 driving the No. 38 Ford, competing for Rookie of the Year. He scored three top-ten finishes and finished 27th in the standings before parting ways with the team that November.
He returned to Cup competition briefly in October 2022, filling in for the suspended Bubba Wallace in the 23XI Racing No. 45 at Homestead-Miami Speedway. Legacy Motor Club signed him to drive the No. 42 full-time beginning in 2024. His debut full-time Cup season produced four top-ten finishes with a best result of sixth at Bristol; he finished 34th in the standings. He improved in 2025, opening the year with a career-best fifth-place finish at the Daytona 500.
In March 2019, Nemechek made his Michelin Pilot Challenge debut at Sebring, sharing a Ford with ThorSport Racing teammates Matt Crafton, Ben Rhodes, and Myatt Snider. In January 2024, he made his IMSA debut at Daytona, co-driving the No. 23 Toyota GR Supra GT4 EVO with Bubba Wallace and Corey Heim.
Nemechek was born the eldest of three siblings and has one older half-brother. He was the subject of a children's book on racing, Racin' Buddies, written by his father in 2001.