Belen Ramirez, 20, of Long Beach was arrested early Saturday morning on suspicion of a felony count of child endangerment, police said.
The one-day-old boy's umbilical cord was still attached when a passerby found him in stroller near a church Tuesday afternoon, not far from the University of Southern California.
Alex Diaz who is a father of 2 children called emergency services with a report of child abuse after first seeing the 'dirty' stroller at the corner of Vermont Avenue and Dana Street on Monday night.
When he passed the same spot the following day around 1pm he decided to look inside, and to his disbelief saw the baby, wearing pajamas and covered in sweat, inside.
'You took your time to dress him up, but you couldn't take your time to take him somewhere safe,' Diaz said.
'You just left him out on the street - threw him out like trash. Can't do that to a baby.'
The baby, described as white or Hispanic, was taken to California Hospital Medical Center for treatment.
The child is in good condition and remains in official custody.
Ramirez remains jailed, it is unclear if she has an attorney, and is being held in lieu of a $100,000 bail.
Los Angeles County's 'Safe Surrender' law allows a parent or guardian to give up a newborn at a hospital, fire station or other approved site if the child shows no signs of abuse.