Hemet police said Ramona Yungaray had not been seen at her home at Royal Holiday Mobile Home Estates at 4400 W. Florida Ave. in several days, and the property manager went to check on her Saturday morning.
The son, Francisco Martinez, answered the door, crying, with dried blood on him, police said.
Detectives questioned Martinez at police headquarters. It was not clear whether he had hired an attorney.
A motive for the killing was not disclosed, nor has the cause of death. An autopsy was pending.
The Associated Press and City News Service contributed to this report.