Authorities say the woman crashed into two parked cars on 5900 block of N. Whitnall Highway shortly after 2 a.m.
"The neighbors after hearing the traffic collision, which was very loud as you can imagine, came out and found the child, the 4 year old hanging in the car seat, " said LAPD Det. William Bustos. "They released her to safety."
Paramedics arriving on scene found the child inside the car but not in the car seat. She told them her mother had run away.
Police said the mother was found about a block away and arrested.
The girl was taken to Providence Saint Joseph Medical Center, where she is expected to be OK. She will be given to child services for the time being.
A nurse who recognized the girl from a previous incident informed police that the woman had a 3-month-old as well. Police returned to the accident scene but found no sign of another child.
Two children, a 3-month-old boy and a 1-year-old girl, were eventually found alone in an apartment in North Hollywood.
Police and children services are investigating before more charges are filed against the mother.
It was earlier reported that the child in the car was a boy, but the child was a girl.