Chastain began his racing career at the age of twelve, competing on local Florida short tracks in both late model and Fastruck Series events, as well as figure eight boat racing. His short track career saw Chastain scoring over fifty wins in feature events, including the 2011 World Series of Asphalt at New Smyrna Speedway, winning three of eight events in the series.
He made his debut in NASCAR national touring series competition in the Camping World Truck Series in 2011, driving the No. 66 Chevrolet for Turn One Racing at Lucas Oil Raceway at Indianapolis, finishing 10th in his inaugural event. Chastain competed in four additional events in the 2011 season, with the 10th being his best finish, in preparation for a full season in the Camping World Truck Series in 2012, competing for Rookie of the Year honors with SS-Green Light Racing. Driving the No. 08 truck sponsored by the National Watermelon Association and National Watermelon Promotion Board, he scored a career-best finish of seventh in the second race of the season at Martinsville Speedway. He bested his 7th-place finish with a 3rd-place finish at Bristol in August.
In January 2013, it was announced that Chastain would drive in 15 Camping World Truck Series races in 2013 for Brad Keselowski Racing. At Iowa Speedway in September, Chastain won his first career Truck Series pole for the Fan Appreciation 200; he led the most laps in the race, finishing second to James Buescher as the race underwent a green-white-checkered finish.
Leaving BKR after the 2013 season, Chastain moved to Ricky Benton Racing for a part-time schedule in 2014. In May, he announced he would make his debut in the Nationwide Series (now Xfinity Series) at Charlotte Motor Speedway, driving the No. 55 for Viva Motorsports. He then joined JD Motorsports in 2015 replacing, Jeffrey Earnhardt. He then finished his best finish of 9th in the season opener at Daytona.