Scott Dekraai, 44, faces a death sentence or life with no parole. The case is currently involved in hearings over alleged prosecutorial misuse of jailhouse snitches, and, a judge could simply decide to sentence him to eight consecutive life terms.
Dekraai could still get a jury trial to decide the penalty.
Orange County prosecutors had refused a plea agreement that would have taken the death penalty off the table. His lawyer announced earlier this week that Dekraai wanted to take responsibility for his actions.
Dekraai, who was arrested a few minutes after the shootings at Salon Meritage in Seal Beach in October 2011, never denied the killings. He told a detective he had been sitting on the beach after a telephone conversation with his ex-wife, Michelle Fournier, about their custody dispute over their son and then drove to the salon.
He described the eight others he shot as "collateral damage." They included one man, David Caouette, 64, who was sitting in his car outside when Dekraai came out of the salon.
The others killed were the salon's owner, Randy Fannin, 62; four employees, Victoria Buzzo, Laura Elody, Michele Fast and Christy Lynn Wilson; and Lucia Kondas, who was getting her hair done.
Dekraai went to the salon carrying multiple guns and wearing a bullet-proof vest.
On Friday, Dekraai responded "guilty, your honor" nine times as the judge asked him how he pleaded. Relatives of the victims packed the courtroom.