The CHP first received reports that the Ford Ranger pickup had left the road on Mount Hamilton Road just south of the observatory at 12:08 p.m.
The driver, a man in his 50s, was a state maintenance employee at the observatory, which is at the mountain's summit, according to the CHP.
He was trapped inside the truck after it fell, and CHP officers were on scene throughout the afternoon partly because the rural mountain road is difficult to access.
The coroner was called at about 1:30 p.m. and the man was pronounced dead at the scene.
