The man, who had not been identified as of 3 p.m. Saturday, had been involved in some sort of altercation with his mother shortly before the stabbing took place, according to initial dispatch reports.
San Bernardino County Sheriff's deputies and paramedics received a 911 call at 10:38 a.m. reporting the stabbing in the 8600 block of Sahara Road in Phelan, according to sheriff's officials.
Authorities learned the 21-year-old had attacked his mother with a knife, said Cindy Bachman, spokeswoman for the sheriff's department.
Several patrol units and a sheriff's helicopter converged on the house and ordered the man out at gunpoint, according to witnesses.
Paramedics transported the injured woman to Arrowhead Regional Medical Center where she was last listed in serious condition, Bachman said.
The woman's son was arrested.
It's unclear what sparked the attacked. The incident is currently under investigation.