It happened sometime overnight on Jefferson Avenue near McCall in Del Ray.
Officers didn't discover the scene until about 4 am Thursday, July 3.
The CHP says a man, later identified as 46-year-old Juan Rios Chincoya, was driving the ATV when he hit a stand-pipe directly in his path and was thrown from the vehicle.
Officers say Chincoya was not wearing a helmet.
He died at the scene.