The incident happened just before 6 a.m. Tuesday at Virgil Avenue and 3rd Street.
Police say the woman was walking in the area when she was struck by a vehicle driven by an 83-year-old man that had just turned north on Virgil.
The driver stopped and got out to try to help her.
Then another vehicle came along and hit them both.
That second driver failed to stop or render aid, continuing to drive north on Virgil, the LAPD says.
Paramedics responded and brought both the 70-year-old woman and the 83-year-old man to a hospital. The woman died from her injuries.
Police are looking for the driver who fled the scene.
Anyone with information is asked to call detectives at (213)473-0234. Anonymous information can be provided to Crime Stoppers at (800)222-TIPS.
The LAPD reminds the public that the city has a $50,000 reward program for providing information that leads to arrests and convictions on serious hit-and-runs .