When deputies arrived, they found a newborn baby boy wrapped in a towel on the front steps. The temperature was 46 degrees, so the deputy took the baby into his patrol car, turned on the heater and called for medical help.
The newborn was taken to Bear Valley Hospital and was then transferred to Loma Linda Medical Center.
The medical staff said he was a few hours old. The newborn is in stable condition in the Neonatal Intensive Care Unit.
He is expected to survive and is in the custody of Child Protective Services.
Authorities said by Saturday morning, they had identified the mother.
She was not placed in custody, but a report has been forwarded to the district attorney's office.
State law allows the parent of a newborn to legally surrender their baby to any employee at any hospital emergency room or designated fire station in California.
Since the law took effect nine years ago, more than 300 babies have been safely surrendered. The church where the baby was left is not a safe surrender site and was not open at the time.