Mason was born into a family with strong roots in motor racing. His father and grandfather designed and built the RTJ Formula Vee — a car his father raced to multiple championships in North America throughout the 1980s and 1990s, competing in the SCCA and ASN Canada FIA. His brother Jesse Mason was also a professional racing driver who finished third in the Indy Lights championship in 2004. The family's technical involvement and competitive history gave Nelson an early immersion in the mechanics and culture of motorsport.
Mason began racing in motocross in 1992 at age four — karting in Canada at that time had a minimum age requirement he had not yet met. By his eighth birthday in 1995, he had won the Ontario Provincial Motocross Championship and the Canadian National Motocross Championship. In 1996, competing in both motocross and karting simultaneously, he added a further provincial motocross title and won the New York State Karting Championship. From 1997 he focused entirely on karting, racing through the World Karting Association and accumulating multiple wins, including international success at the 1999 Constructor's Cup.
In 2002, Mason moved up to the SKUSA ProMoto Tour in the 80cc Shifterkart class with MBA Karts. The following year with First Kart North America he competed in the Champ Car Stars of Tomorrow Karting Championship, winning on his first outing and qualifying pole and winning the Pre-Final at the Champ Car World Finals in California, before finishing third in the Final and the overall championship. Between 2006 and 2008 Mason also competed in the British Universities Karting Championship for Oxford Brookes University, finishing third, second, and third in the championship across those three seasons.
Mason made his open-wheel debut in his father's Formula Vee in SCCA competition in 2004, limited by budget to a handful of rounds but managing a podium at Pocono Raceway. In 2005, he competed in the Walter Hayes Trophy at Silverstone in a Formula Ford prepared by himself and his brother, fighting through rain and a field of over 150 cars to reach the final. An incident-free race from the back saw him advance rapidly in the early laps before losing third place near the finish.
In 2006, back in North America, Mason raced Formula Vee in the SCCA, collecting wins, podiums, and lap records on his way to third in the championship at the SCCA Runoffs. He returned to European competition with appearances at the Formula Ford Festival in 2007 and 2008, with a best result of 21st in 2007 after being eliminated from a potential top-ten in the semi-final. He also competed in six rounds of the 2010 Italian Formula Three season with Team Ghinzani.
After a three-year break from European competition, Mason returned in 2013 to the European F3 Open Championship with Team West-Tec. He took wins at Le Castellet and Jerez along with three further podiums, finishing third in the series standings. At the end of the year he claimed the European F3 Open Winter Series Championship, a title that opened doors to higher-profile European single-seater machinery.
In 2014, Mason graduated to the GP3 Series with Hilmer Motorsport, stepping into one of the primary feeder categories for Formula 1 at the time. He subsequently competed in the World Series Formula V8 3.5 with Teo Martín Motorsport, a championship that featured a number of drivers from the wider European junior single-seater ecosystem.
Nelson Mason's career represents one of the less common paths through North American grassroots motorsport — motocross champion to karting to Formula Vee — into the European junior single-seater ranks, where he won championships and raced at the GP3 level. His background in a family that built and raced its own cars, alongside his own progression across multiple categories and continents, marks him as an unusually self-determined figure in the landscape of North American drivers who sought the European route.