The jury reached the verdict after deliberating two days in Marin County Superior Court.
Deputy District Attorney Charles Cacciatore said the jury did not convict Mitchell of the special circumstance of murder during a kidnapping.
Mitchell, 29, faces 34 years to life in prison when he is sentenced Aug. 16, Cacciatore said.
Cacciatore said the verdict reflects "the seriousness with which we treat domestic violence cases here in Marin County."
"We want to make sure the victims take steps to get out of violent relationships and know they will have the assistance of the Marin County courts when they make that decision," Cacciatore said.
Mitchell, an heir to the Mitchell family pornography and strip club business, was charged with killing Keller by repeatedly hitting her head with a softball bat at her Diablo Court home in Novato and kidnapping their daughter on the toddler's first birthday on July 12, 2009.
He also was convicted of stalking, child endangerment and domestic violence, Cacciatore said.
Mitchell testified he fought off two men who actually killed Keller and tried to take their daughter from the residence as he arrived to celebrate the girl's birthday.
He was arrested about five hours later in Citrus Heights after he ran out of gas. His daughter was not injured.
The prosecution claimed the slaying was the culmination of a series of domestic violence incidents committed by Mitchell against Keller.
The verdicts also came four years to the day after Mitchell's father Jim died.
