Rene Brunet, 63, was participating in the Mt. Diablo Trail Run when he died, a coroner's deputy said.
Brunet was found down an embankment on Mitchell Canyon Fire Road at about 7:30 p.m. by park staff and the race director, Mt. Diablo Supervising Ranger Dan Stefanisko said.
He was overdue to finish the race, which prompted the search, Stefanisko said.
The race began at 8:30 a.m. and participants can run an 8, 25 or 50 kilometer race.
A cause of death for Brunet has not yet been determined, the deputy said.