The ambulance reportedly T-boned the vehicle near Orange Avenue and Spring Street at 4:36 a.m.
Three people, including a patient, traveling in the ambulance were injured. The driver of the sedan was also injured.
All four occupants were transported to Long Beach Memorial Hospital for treatment. The patient in the ambulance was listed in critical condition prior to the crash.
Investigators say the ambulance's siren and lights were not on because they were transporting the patient to another hospital.
The intersection was closed for about three hours following the crash.
The Los Angeles County Fire Department and Signal Hill Police Department were leading the investigation. It remains unknown who ran the red light.