Heylen began karting in 1992 and progressed through a series of British and European single-seater championships, racing in British Formula Ford (2001), where he won the Formula Ford Festival at Brands Hatch in 2002, Formula 3 Spain and Euro (2003), and German Formula 3 (2004), finishing third in that championship with six wins and eight podium finishes. In 2005, he was crowned Eurocup Megane Trophy champion after earning seven wins in the series.
He made his first venture into sports car racing in Europe around this period, winning the 24 Hours of Zolder in 2005 with Selleslagh Racing Team. He later competed in Belcar GT at Spa-Francorchamps, again securing victory. In 2008, he returned to the 24 Hours of Zolder alongside the future Porsche factory driver Laurens Vanthoor. His European sports car involvement continued through 2011, when he ran a full season in FIA GT with Prospeed Competition, partnering Petri Lappalainen.
Heylen raced in the 2006 Champ Car World Series for Dale Coyne Racing, partnered with Cristiano da Matta, scoring a fifth-place finish at the Cleveland Grand Prix — the team's best result at the time. He returned for part of the 2007 season with Conquest Racing, coming close with a second-place finish at TT Circuit Assen before being replaced by a funded driver.
In 2010, he made starts in the Indy Lights season with Team E, earning a second-place finish in the opener at St. Petersburg.
Heylen's long-term future lay in North American GT racing. He began competing in Grand-Am in 2008, driving a Porsche 911 to sixth in class at the 24 Hours of Daytona for Synergy Racing. After intermittent North American appearances between 2012 and 2013 — including three podiums in the American Le Mans Series with JDX Racing alongside Mike Hedlund — he established himself as a consistent frontrunner.
The 2014 TUDOR United SportsCar Championship season proved a turning point. Running with Wright Motorsports/Snow Racing in a Porsche 911, Heylen co-drove with Madison Snow and Marco Seefried to a third-place finish at the Rolex 24 at Daytona, one of only two GTD entries to use only three drivers for the entire race. Further podiums followed at Road America and at Petit Le Mans with actor Patrick Dempsey as a third driver. The following year, 2015, brought a second consecutive Rolex 24 podium alongside Dempsey and Phillip Eng.
In 2017, paired with Michael Schein in the Pirelli World Challenge Sprint X GT Pro/Am championship for Wright Motorsports, Heylen and Schein earned three wins and seven podiums, finishing second in both the team and driver championships.
Heylen's most dominant season came in 2021, entirely within Wright Motorsports machinery. In Michelin Pilot Challenge, partnered with Ryan Hardwick, the No. 16 Wright Motorsports Porsche claimed three victories including a decisive final-round win at Road America, earning Heylen the Driver Championship and Wright the Team Championship. In the IMSA WeatherTech SportsCar Championship, he joined Patrick Long and Trent Hindman for the four endurance events, earning a second-place finish at the Twelve Hours of Sebring and ultimately securing the Michelin Endurance Cup as a team.
In GT World Challenge America, he reunited with Fred Poordad for a full season in the No. 20 Porsche 911 GT3 R, finishing in the top five in all thirteen races and claiming the Drivers' Championship, with Wright taking Team Championship honors. Combined with additional titles from the endurance subseries, Heylen and Wright Motorsports accumulated nine championship titles during the 2021 season.
This achievement earned Heylen the Porsche Cup award, an annual honor bestowed on the most successful private Porsche racing driver worldwide since 1970, accompanied by a new Porsche road car.
Heylen has also contributed to driver development. In 2008 and 2009, he served as chief driving instructor for the Volkswagen Jetta TDI Cup, overseeing the training of thirty drivers between the ages of seventeen and twenty-six. That season was documented in a reality television series titled Racing Under Green. In 2019, he served as Race Director for the Mazda Road to Indy's USF2000 series.