The crash happened at about 4:20 p.m. on Balfour Road east of Sellers Avenue.
According to witnesses, the two girls had been speeding when, for an unknown reason, their vehicle drifted to the right.
When the driver, a Brentwood resident, tried to correct the vehicle and bring it back onto the road, she lost control, crossed both lanes of traffic and crashed into a utility pole on the opposite side of the road, Maguire said.
Emergency crews had to extricate both girls from the vehicle before they could be airlifted to John Muir Medical Center.
The passenger suffered a broken pelvis, broken finger, a punctured lung and lacerations, Maguire said.
The driver, whose injuries are potentially life-threatening, suffered a fractured skull, fractured pelvis, fractured femur, bleeding on the brain and lacerations, Maguire said.
The cause of the crash remained under investigation today, Maguire said.