The suspect - wearing shorts, no shirt and with a shaved head - barricaded himself inside the Family Dental Care office after the Wednesday morning shooting, police Sgt. Karl Becker said. He surrendered after a hostage negotiator coaxed him out about an hour after the shootings.
Detectives did not release a motive or identify the suspect. But the Ventura County Star newspaper reported that a dental office worker said the gunman was married to the slain victim.
A fourth person was grazed by a bullet in the attacks.
In the minutes following, police evacuated more than a dozen people from the dentist's office and a chiropractic office next door, including two children who witnessed the shooting.
Carley Hodgson, an X-ray technician intern working next door, said she heard three shots in quick succession and huddled in the back room with six co-workers behind a locked door for about 30 minutes.
Shawn Peloquin, a mechanic who owns an auto repair shop behind the dentist's offices, saw people running out the back and into his driveway, including a man who had a stomach wound.
Peloquin said the injured man told him that his sister had been shot several times.
Peloquin and Becky Brown, a volunteer EMT, tried to stop the bleeding by applying pressure to the wound.
"He kept asking, 'Am I going to die?"' Brown said.
Simi Valley is about 35 miles northwest of Los Angeles.