Cassill began racing on a quad at age three and progressed to go-karts, winning four International Kart Federation championships on both dirt and asphalt surfaces. In 2000, at the age of ten, he won three state championships in three different classes on the same night. At age sixteen, he became the youngest winner in ASA Late Model Series history with a victory at Lake Erie Speedway on June 9, 2006 — a record that stood until Erik Jones surpassed it at age fourteen in 2010. A second ASALMS win followed weeks later at South Georgia Motorsports Park, making him the first driver to win in both the Northern and Southern Divisions. He later became the first to win in all three ASALMS divisions when he won the Challenge Division at I-70 Speedway.
Cassill was signed by Hendrick Motorsports in December 2006 after impressing in a GM Racing Development evaluation process. He made his Nationwide Series debut at Gateway International Speedway in July 2007 at the minimum age of eighteen, finishing 32nd. In 2008 he drove for JR Motorsports and Phoenix Racing in sixteen Nationwide races, earning five top-tens, a first pole at Loudon, and Rookie of the Year honors. He also competed in a limited Truck Series schedule for Randy Moss Motorsports that year. He made his Cup debut at Michigan in 2010, driving for veteran team owner James Finch.
Cassill's Cup career was defined by his work with small, underfunded teams. He drove for BK Racing in 2012 after the team purchased the assets of the former Red Bull Racing program. In 2013, he joined Circle Sport and later drove the jointly fielded No. 40 for Circle Sport and Hillman Racing. The 2014 season marked his career-best individual result: a fourth-place finish at the Geico 500 at Talladega — the best of his Cup career. He also tied his previous best of twelfth at the Daytona 500 that year.
Cassill joined Front Row Motorsports for 2016 and 2017, with 2017 representing his most complete Cup campaign — a full chartered season with TriStar Motorsports in the No. 72. At the 2017 Daytona 500 he qualified seventeenth, led three laps in the final stage, and finished eighteenth. He also delivered a notable run at Indianapolis, finishing twelfth by keeping the car clean while others crashed around him. He stepped back to a part-time role at TriStar in 2018 before his final Cup appearance at ISM Raceway that November.
Cassill returned regularly to the Xfinity Series throughout his career. He drove for JD Motorsports, Shepherd Racing Ventures, and ultimately Kaulig Racing, where he drove the No. 10 Chevrolet Camaro in 2022. His best Kaulig run came at New Hampshire, where he led seventeen laps and appeared to finish third before being disqualified in post-race technical inspection. Sponsorship from Voyager Digital supported that deal, but when Voyager filed for bankruptcy mid-season, Cassill's tenure with Kaulig ended. He was not retained for 2023.
In May 2024, Cassill paid $25,000 as part of a broader $2.42 million settlement alongside Rob Gronkowski and Victor Oladipo to resolve a class-action lawsuit filed by Voyager Digital investors.
Cassill is emblematic of a generation of NASCAR drivers who competed at the Cup level primarily through self-funded and sponsor-dependent arrangements, often with small teams operating well outside the resources available to charter holders. Despite limited equipment, he compiled a respectable safety record — logging the lowest crash rate among Cup drivers in 2019 — and demonstrated real speed on superspeedways, where aerodynamic equality partially leveled the playing field between large and small teams. His karting background and early short-track dominance established him as a genuine talent who faced structural barriers to higher-level success throughout his career.