Police Cpl. Anthony Bertagna says Rubi Rubio briefly held onto the trunk of the car before falling off and hitting her head last Thursday in Santa Ana.
Rubio died from her injuries on Saturday while surrounded by her family at a hospital.
Investigators are asking for the public’s help in finding the robber.
Police say the man forcibly took the phone from the teen while she was walking her 7-year-old sister home from school. He ran to the car, which was driven by another person, with Rubio giving chase.
Bertagna described the vehicle as a light gray or silver Pontiac.