Coby started racing in quarter midgets before moving up to pro stocks, late models and SK modifieds. He won two Whelen All-American Series championships at his home track, Stafford Motor Speedway. He has also dabbled in open-wheel midget racing. In 2002, Coby debuted in the NASCAR Whelen Modified Tour at Stafford and ran part-time in the series for a number of years. He won his first NWMT race at Stafford in 2006 and continued to run part-time on the tour for the next half-decade, earning his second career win at Thompson Speedway Motorsports Park in 2011. He also ran in the Valenti Modified Racing Series during that time. The win at Thompson came with car owner Wayne Darling; Coby won his first NWMT championship with Darling in 2012 on the strength of five race wins. The following year, Coby was second in points and made the decision to switch teams to Mike Smeriglio Racing for the 2014 season. He won his first race with the team, the Battle at the Beach, and claimed the 2014 NWMT championship. In 2015, Coby entered the NWMT season finale at Thompson Speedway Motorsports Park tied with Ryan Preece atop the point standings after Coby had controversially spun Preece for a race win at New Hampshire Motor Speedway in the later stages of the season. Coby won the race and the championship after a mid-race pass onJustin Bonsignore for the lead. For the 2016 season, Coby claimed nine NWMT pole awards and claimed his third consecutive tour championship, becoming the first driver in tour history to accomplish such a feat. Following the first two races of the 2017 NASCAR Whelen Modified Tour season, Coby was 16th in the point standings but rallied to claim the championship by the end of the season. He also ran his first K&N Pro Series East race, driving for Calabrese Motorsports at Thompson Speedway Motorsports Park. In 2018, Coby finished third in the NWMT point standings while rival Justin Bonsignore won the championship. Coby won two races the following year and claimed his sixth title by nine points over Bonsignore. He also ran a limited stock car schedule, winning a feature at the World Series of Asphalt. Coby won another NWMT championship in 2019, using a large accumulated points lead to withstand a flat tire suffered in the season finale. After the championship, Coby's car owner, Mike Smeriglio, decided to sell his team, leaving Coby without a ride. In the following offseason, Coby bought one of Smerigilio's three cars and started Doug Coby Racing with his crew chief, Phil Moran.
