The driver of the other car, a woman in her mid-40s, was seriously injured and rushed to the hospital.
Officers and medics responded to the scene, the 17100 block of Highway 99, at about 10:30 a.m. after receiving a report of a serious accident.
A preliminary investigation found that the woman in her 70s was driving south on Highway 99 when her car started to veer into the northbound lanes, then slammed head-on into an oncoming car.
The older woman was pronounced dead at the scene. The woman in her 40s, in the northbound car, was taken to the hospital with serious injuries.
There were no passengers in either vehicle.
Police said it is not known what caused the woman to veer into oncoming traffic. The roadway at the time of the crash was bare and dry.
Northbound lanes of Highway 99 were closed at the scene during the investigation and are expected to reopen at about 1 p.m.