San Diego County jurors also found Shelley Malil guilty of assault with a deadly weapon. He was acquitted of residential burglary.
The panel began deliberations Wednesday in Vista Superior Court after a three-week trial.
Malil -- who played Haziz, a co-worker of Steve Carell's character in "The 40-Year-Old Virgin -- could faces 21 years to life in prison for the 2008 stabbing of Kendra Beebe.
The mother of two was stabbed more than 20 times by Malil during a violent quarrel at her San Marcos home.
Defense attorney Matthew Roberts told jurors in his closing arguments there was no doubt Malil went too far when he picked up the kitchen knife after finding Beebe on her patio with another man, but Malil never intended to kill Beebe.
Deputy District Attorney Keith Watanabe countered that Malil, 45, brought the knife to the home and did not grab it in the heat of the moment.
Watanabe showed jurors photographs of deep wounds on the victim's face, neck and torso and told them they did not have to determine a motive.
Watanabe accused the defense of trying to taint the image of Beebe so the jury would acquit the actor.
Beebe was lucky to be alive, Watanabe said.