Coby was born in Connecticut and began his racing career in quarter midgets before progressing through pro stocks, late models, and SK Modified division racing. He won two Whelen All-American Series championships at Stafford Motor Speedway, his home track, and dabbled in open-wheel midget racing before focusing on the Whelen Modified Tour full-time.
Coby made his Whelen Modified Tour debut at Stafford in 2002 and ran part-time in the series for several years while also competing in the Valenti Modified Racing Series. He scored his first tour victory at Stafford in 2006. His second career win came at Thompson Speedway Motorsports Park in 2011, working with car owner Wayne Darling. That partnership produced his first championship in 2012, backed by five race wins across the season.
For 2014 Coby switched to Mike Smeriglio Racing and won his first race with the team at the Battle at the Beach. He claimed the 2014 title with Smeriglio before defending it the following year in dramatic fashion. Entering the 2015 season finale at Thompson tied in points with Ryan Preece — a rivalry that had been inflamed when Coby controversially spun Preece for a win at New Hampshire Motor Speedway — Coby won the race and the championship with a mid-race pass on Justin Bonsignore.
The 2016 season brought nine pole awards and a third consecutive title, making Coby the first driver in tour history to accomplish a three-peat. He won his fourth championship in 2017 despite starting the season 16th in points after two rounds. He and Bonsignore engaged in a sustained championship rivalry in this era: Bonsignore won the 2018 title while Coby finished third, but Coby responded by winning his sixth championship in 2019 by nine points over Bonsignore, helped by a large accumulated points cushion that absorbed a late-season flat tire.
After the 2019 championship, Smeriglio sold his team. Coby purchased one of his cars and began running Doug Coby Racing with crew chief Phil Moran.
Coby has also competed in other series throughout his career. In 2017 he ran a K&N Pro Series East race at Thompson, finishing fourth. His most notable venture outside of the Modified Tour came in 2021 when he entered the inaugural Superstar Racing Experience event at Stafford Motor Speedway. He won the second heat and the main event in front of his hometown crowd, a performance that led directly to a Camping World Truck Series start with GMS Racing at the Bristol night race in September 2021 — his first appearance in one of NASCAR's top three national series.
Coby joined Tommy Baldwin Racing in 2022 and won his first start with the team at Riverhead Raceway. A partial season with TBR helped the team claim the owners' championship. In 2023 he signed for a full TBR schedule, but the team's season was disrupted when owner Tommy Baldwin Jr. was diagnosed with cancer and suspended operations, returning only for the Monadnock Speedway race in September.
Outside of racing, Coby worked in financial services before founding Rescue Dog Realty, a real estate company structured to donate commission earnings to animal rescue shelters.
Doug Coby's six Whelen Modified Tour championships and his record of three consecutive titles place him among the all-time greats of New England short-track racing. His longevity, consistency, and ability to win championships with multiple teams across more than a decade define a career that has made him one of the most recognized names in the Modified division.