Erik began racing at the age of 12 in the River Vally Kart Club. He won the championship in the purple plate class in his second year of competition, later racing Allison Legacy Series cars after go karts. His first year of super late models was at Illiana Motor Speedway, with Erik finishing 3rd in the final standings with 1 win. Erik beat the best Wisconsin super late model drivers to win the 2003 Wisconsin Challenge Series championship. At that time his five wins were the most in the series' history. He set the super late model track record at Lake Geneva Raceway in 2004.
Darnell drove in his first NASCAR Craftsman Truck Series race at Milwaukee in 2004, finishing 26th. He also raced in numerous NASCAR touring series races.
Darnell drove in several NASCAR touring series and six ARCA races in 2005.
Darnell raced full-time in the Craftsman Truck Series (CTS) in 2006. He had twelve Top-10 finishes in 25 events, and he was the series' Rookie of the Year. He continued to drive for the team in 2007, as well as testing the team's Busch Series cars. On April 28, 2007 Darnell won the O'Reilly Auto Parts 250 at Kansas Speedway for his first Craftsman Truck Series win.
Darnell started the 2008 season by capturing the pole position at the Daytona International Speedway. He won his second career CTS race in June 2008 at Michigan International Speedway by .005 of a second over Johnny Benson.
For 2009, Darnell planned to compete in 15 NASCAR Nationwide Series races, sharing the car with Cup Series driver David Ragan. Darnell ran for Rookie of the Year and Darnell ran his first of 15 NASCAR Nationwide Series race at Richmond International Raceway where he finished 12th. Also, Darnell competed in seven of the final 11 NASCAR Sprint Cup Series races. He ran the car at Atlanta, Loudon, Kansas, Talladega, Texas, Phoenix, and Homestead. Darnell competed in the #96 Academy Sports + Outdoors Ford for the Atlanta, Talladega, and Texas races. He alternated the ride with Bobby Labonte.
In 2010, Darnell found himself without a ride for most of the season due to the lack of sponsorship. He continued to stay on at Roush Fenway Racing as a practice-qualifying driver for Carl Edwards at Standalone Nationwide Series races. Darnell did a 3 race deal to drive the #16 Ford in the Nationwide Series. His best finish was 14th at Dover and Texas.
In 2011, Darnell returned to the Cup Series, driving for Whitney Motorsports in several races; for 2012, he competed for Key Motorsports in the Nationwide Series.
Darnell didn't race in NASCAR during 2013. He won the Dick Trickle 99 Super Late Model Oktoberfest race at LaCrosse Fairgrounds Speedway.
(key) (Bold - Pole position awarded by qualifying time. Italics - Pole position earned by points standings or practice time. * – Most laps led.)
1 Ineligible for series points