Smithley was born in Pennsylvania, raised partly in Virginia, and later in Georgia. He began racing in 2007 during his sophomore year of high school, competing in Bandolero cars at Atlanta Motor Speedway and Lanier National Speedway and winning Rookie of the Year in 2008. He credits his participation in high school theater — including a lead role in a production of Charlie and the Chocolate Factory — as the source of confidence that enabled him to take risks on track.
After Bandoleros, Smithley progressed to Legends cars with support from Peachtree City Golf Carts, a sponsorship that emerged after his original cart was stolen. He was selected for the Richard Petty Driver Search program in his first year after high school. Growing up, he idolized Dale Jarrett and Carl Edwards.
Smithley made three ARCA Racing Series starts in 2013 driving for Derrike Cope, with his Pocono start being his first stock car race. He made one ARCA start in 2014 for Wes Gonder, finishing eighteenth at Talladega, and one more in 2015 for Rick Ware Racing, where engine problems cut his run short.
Smithley debuted in the Truck Series at Atlanta Motor Speedway in MB Motorsports' No. 63 Chevrolet Silverado, recording a high finish of fourteenth at Michigan across four 2015 starts. He returned for a single race at Atlanta with MB Motorsports in 2016. In March 2019, he drove for Chad Finley Racing at Texas Motor Speedway as part of a personal goal to compete in all three national NASCAR series in a single weekend. The following year he signed with Niece Motorsports for the Charlotte Truck race.
Smithley made his Xfinity debut at Homestead in the final race of 2015, driving Derrike Cope's No. 70 Chevrolet. In February 2016, JD Motorsports announced he would drive their No. 0 Chevrolet on what began as a three-race deal but expanded to nearly the full season. He finished eighteenth in points and scored top-fifteen finishes at both plate tracks, including at Talladega, though road course incidents — such as running the bus stop at Watkins Glen and causing an engine failure — tempered the results.
Smithley returned full-time to JD Motorsports in 2017 and scored his first two series top-ten finishes, at Daytona and Iowa, and led his first Xfinity Series lap at Talladega. By his own characterization, the rest of that season was a "sophomore slump." In addition to driving, Smithley operated as his own publicist during this period, calling potential sponsors directly before each race week.
His 2018 Daytona result — a fifth-place finish — was his first career top-five and came after overcoming a spin during the race. He also changed crew chiefs, bringing Wayne Carroll aboard. At the 2019 Darlington throwback race, Smithley drove a tribute scheme honoring MB Motorsports owner Mike Mittler, who had died earlier that year and given Smithley his first Truck Series opportunity.
In 2020, Smithley joined SS-Green Light Racing as part of a Rick Ware Racing alliance. He continued to race on a part-time and intermittent basis through 2022 and 2023, moving between JD Motorsports, DGM Racing, B. J. McLeod Motorsports, and Alpha Prime Racing.
For 2025, Smithley was announced as the full-time driver for SS-Green Light Racing with BRK Racing in the No. 14. He ceded his seat at Circuit of the Americas to Carson Hocevar to help the team qualify and returned to the car at Phoenix. For 2026, Smithley was set to return to SS-Green Light with BRK Racing, now in the renumbered No. 0.
Smithley made his Monster Energy NASCAR Cup Series debut at Michigan in June 2018 with StarCom Racing in their No. 99, retiring after one lap with a transmission problem. He made two additional Cup starts that season with Premium Motorsports.
In 2019, Smithley picked up a part-time ride with Spire Motorsports in the No. 77 Chevrolet, starting at Atlanta. His best result of the year came with a 28th-place finish at the Brickyard 400. While driving Rick Ware Racing's No. 52 Ford at Las Vegas, he was publicly criticized by Kyle Busch following racing contact.
Smithley continued Cup appearances with RWR through 2020, 2021, and 2022 on partial schedules. In 2021, he substituted for Cody Ware in the No. 51 at the Richmond playoff race after Ware was sidelined with carbon monoxide poisoning following Darlington. Smithley also filled the No. 13 for MBM Motorsports in the 2021 Daytona 500 attempt after that team needed a driver with confirmed sponsorship.
He competed in the 2025 Cook Out Clash at Bowman Gray Stadium in the No. 66 Ford for Garage 66, MBM's Cup Series rebrand.
Smithley's father and both grandfathers served in the military. Outside racing, Smithley has expressed ambitions to become a television announcer. He has cited his drama and theater background as foundational to his ability to perform under pressure — both in the paddock and on the racetrack.