Keen entered professional racing in 2004, joining the Rolex Sports Car Series with Autometrics Motorsports in the Super Grand Sport class, collecting two third-place podium finishes from three starts at Virginia International Raceway and Barber Motorsports Park.
In 2005, he ran the full Rolex Series schedule and scored his first professional victory at the Six Hours of Watkins Glen. He ended the season fifth in the standings alongside co-driver Cory Friedman, with eleven top-ten finishes.
From 2006 onwards Keen rotated between Synergy Racing, Farnbacher-Loles, and Autometrics in various Porsche GT3 variants, building a record for consistency and pole-position pace. During the 2007 Rolex 24 at Daytona, his Farnbacher-Loles number 85 Porsche led the GT class for 22.5 hours before an engine failure ended the run.
In 2008 he claimed a win at Mid-Ohio Sports Car Course with Farnbacher-Loles in wet conditions, along with one pole position and five top-five finishes.
Keen and co-driver Dirk Werner won the Grand American Rolex GT Championship in 2009, clinching the title with 359 points each. The pair won four races — at New Jersey, Mid-Ohio, Daytona, and Watkins Glen, three of them in succession — and secured three pole positions and seven podiums across the season.
Keen switched to a Mazda RX-8 with Dempsey Racing for the Rolex Series in 2010, delivering the team's first and only series victory alongside James Gue at Watkins Glen International Raceway. He also campaigned a Porsche for Alex Job Racing in the American Le Mans Series, where he took pole, set the ten fastest race laps, and won at the 12 Hours of Sebring.
That year he also raced in Europe with Farnbacher Racing, claiming a class win and second overall at the 24 Hours of Nürburgring and a second place in the GT Pro class at the 24 Hours of Le Mans.
Keen joined Brumos Racing for 2011, partnering with Andrew Davis in the famous number 59 Porsche 911 GT3. The pair won the Six Hours of Watkins Glen and Laguna Seca, securing the 2011 Grand-Am Rolex GT Championship in what was Brumos's first year back competing at the GT level. Keen also ran the American Le Mans Series concurrently with Alex Job Racing, scoring multiple podiums.
Returning to Brumos Racing alongside Davis in the Rolex Series, Keen also took up a full endurance campaign with Alex Job Racing's WeatherTech Porsche in the American Le Mans Series alongside Cooper MacNeil. The partnership produced the GTC Drivers' Championship and team championship for Alex Job Racing, with wins at Lime Rock Park and Virginia International Raceway. It was Keen's third championship in four seasons.
Keen continued with Alex Job Racing into the IMSA WeatherTech Series era. After the Canadian Tire Motorsports Park round of the 2016 season he was released by team owner Alex Job, replaced by Porsche Junior driver Sven Mueller. At the time of his departure, Keen sat eighth in the driver standings.
In February 2021, Keen set a Guinness World Record for the fastest speed achieved by a vehicle indoors, reaching 102.65 mph (165.20 km/h) inside the New Orleans Convention Center in a Porsche Taycan Turbo S. The record had previously stood at 86.99 mph, set in 2013 by Mikko Hirvonen inside the Helsinki Expo Centre. Keen's record was subsequently broken in July 2023 by Jake Hughes driving a Formula E-derived car at the ExCeL Centre in London.
Keen's three Grand-Am GT championships across 2009, 2011, and 2012 place him among the most successful drivers in the history of that series. His versatility across endurance formats — Grand-Am, ALMS, and European GT events — and his consistent Porsche results made him a significant figure in North American sports car racing during the late 2000s and early 2010s.