The crash happened at the intersection of Garfield and Main avenues in Alhambra about 2:30 a.m.
Alhambra police say each officer was driving alone in their unit. They say police were investigating a possible robbery in the rear parking lot of 100 North First Street when the police cars collided with each other.
Paramedics rushed both officers to Huntington Memorial Hospital. Officer Ryan Stringer, 26, died from his injuries. The other officer, whose name has not been released, was in stable condition and is expected to survive.
"The department is really grieving today, as is the profession for the loss of this officer," Alhambra Police Chief Mark Yokoyama said. "Officer Stringer was a very energetic, very friendly police officer, very well-liked by other members of the department and the community."
Witnesses said one officer ran a red light and T-boned the other.
"It was pretty bad," said one witness who declined to identify himself. "I pulled over to render assistance for the closest police officer that was to me when I parked. That's when I observed the officer bleeding pretty profusely."
Stringer had been with the department for two years. Just over a year ago, Stringer was in a motorcycle accident on the 605 Freeway while he was on his way to work.
"He was not expected to live at that time. They told us he had no chance, then he came back 100 percent. In six months he was back to work full time," said Stringer's father, Sonny. "He just loved everybody. He couldn't hardly wait. He'd be so excited about going to work he'd be there an hour, an hour and a half early."
"To me, it's a miracle that he was out there being able to come from one extreme and build himself back up and going out there and serve the community," said Paul Young, a neighbor.
The California Highway Patrol is investigating the collision. Police and the CHP have given no further details on the cause or circumstances of the crash.
The Associated Press contributed to this report