The car burst into flames Wednesday after crashing through a fence and into a tree near Harmon's home in Brentwood, said fire department spokesman Brian Humphrey.
It was not clear what caused the accident.
Harmon, who played a doctor on NBC's medical drama St. Elsewhere from 1983 to 1986, ran from his house to help and rescued the teens before authorities arrived. A neighbor had called the fire department.
"He was there almost immediately. These boys certainly owe their lives to the quick and selfless action of Mr. Harmon," Humphrey said.
One teen was listed in critical condition with serious burns, a broken arm and a cut on his face. The other had less serious injuries, Humphrey said.
Their names and ages were not released.
Local TV stations said the actor didn't want to comment.