Richard Buck, winner of five Indianapolis 500s as a crew chief and most recently the vice president of racing operations for the International Motor Sports Association, will take over for Darby after the Rolex 24 at Daytona at the end of the month.
Darby, who has been Cup director for the last 12 years, will remain managing director of competition for NASCAR.
"What we've seen over the past few years is there needs to be a better process," said NASCAR President Mike Helton. "That better process is going to give us the ability to make a better product on the racetrack that the fans expect to see. John will still be a big part of all of that. Richard will be more visible and be the face of the Sprint Cup Series sporting part of NASCAR."
Buck spent last year with IMSA and as managing director of NASCAR's Touring Series, proving a "deep and unique knowledge of motorsports, and his versatility and innovative approach to competition should serve the NASCAR Sprint Cup Series garage extremely well," Helton said.
He worked from 1980 to 2000 as a crew chief, team manager and director of operations with Champ Car teams, and won five Indy 500s as a crew chief with Rick Mears, Al Unser Sr., Al Unser Jr. and Emerson Fittipaldi. He worked with teams in all three NASCAR national series before joining NASCAR in 2005.
"I'm eager and ready to pursue this unique opportunity to serve as the NASCAR Sprint Cup Series managing director," said Buck. "I appreciate the faith that NASCAR's management team has entrusted in me for this role. As we prepare to launch the 2014 NASCAR Sprint Cup Series season, I'm thrilled to expand my role in the sport of NASCAR, and I look forward to helping the NASCAR Sprint Cup Series competition team continue to produce the best racing in the world."
IMSA will realign its competition structure after the Rolex 24 Hours and Buck's move to NASCAR. Scot Elkins, IMSA vice president of competition and technical regulations, will work closer with NASCAR's top executives in racing to form a joint NASCAR/IMSA Competition Committee. The goal will be to share ideas, best practices and resources on innovation and development regarding the two different types of racing.
Brad Moran will replace Buck as managing director of the Touring Series.
Darby, a fair but no-nonsense official, oversaw the implementation of the "Car of Tomorrow" in 2007 and the "Gen-6" car last season.