Officials say vehicle was westbound on Cypress Avenue when it went out of control near Azusa Canyon Road about 12:15 a.m.
It plunged about 200 feet into the water and firefighters rescued the critically injured driver and a passenger.
"It was steep terrain, it was about 150 to 200 feet over the side," said Los Angeles County Fire Battalion Chief Scott Fellhoelter. "No access into the quarry, so we're having to airlift our own folks. We got them down in there and the victims extricated."
Investigators are trying to determine how the car lost control.
City News Service contributed to this report.