Jose Escobar Monterrozo, 36, also pleaded guilty to two felony counts of child abuse and residential burglary, which were charges related to the homicide.
Initially, Escobar Monterrozo was charged with murder. Those charges were reduced after the District Attorney's Office received additional evidence about the case through its investigation, according to prosecutor Rameen Minoui.
In an interview, Minoui said reducing the charge from felony murder to voluntary manslaughter was the right thing to do in this case.
"It was an easy decision. It was a right decision," Minoui said.
Attorney Justin Tuttle, who is with the Public Defender's Office, had argued during the preliminary hearing that this was a "classic" voluntary manslaughter case, saying that the slaying was committed in the heat of passion.
"I am pleased with the result," Tuttle said today.
He said Escobar Monterrozo was "shocked and saddened" about the slaying.
Tuttle said Escobar Monterrozo faced 25 years to life if he had been convicted of murder.
Ventura County Superior Court Judge Kevin McGee set sentencing for Jan. 17.
Escobar Monterrozo killed Claudia Minjivar, 33, on Jan. 25 at the couple's Simi Valley home.
Escobar Monterrozo used a car steering wheel lock to beat Minjivar, who was involved in a romantic relationship with an old boyfriend, according to court testimony.
The couple's 5-year-old son was there during the slaying, and Minjivar's 15-year-old daughter arrived later from school, according to testimony from Escobar Monterrozo's preliminary hearing.
Escobar Monterrozo, who is from Guatemala, is facing a deportation hearing after his release from prison, according to jail records.