He made his first career start in the series in 2000 with a fifth place finish in his class. The following year, he earned a class victory at the Circuit Trois-Rivières. He also competed at the KONI Challenge Series and earned a class victory at Watkins Glen International. In 2002, he won the KONI Championship in the ST Class, despite not winning a single event. He won six races in the GSI class the following year and captured the title. In 2004, he competed in the Rolex 24 at Daytona for the SGS class and won, along with eight other starts in the series. Then in 2007, he captured a GT victory at the same event at Daytona. Dumoulin would drive 11 more races and finished ninth in the standings. In the KONI Series, he won at Watkins Glen International in one of his three starts. For 2008, he co-drove for the #15 Ford Mustang Cobra team of Blackforest Motorsports along with Tom Nastasi, David Empringham and Boris Said, Dumoulin drove two races that season (Daytona and Montreal).
Dumoulin competed at the 2009 NAPA Auto Parts 200 at Montreal for the #23 car team for R3 Motorsports; he finished in seventh place with a largely damaged car, including a busted radiator. It was also the first top 10 finish in the team's history.