Under a plea deal, rape, robbery and kidnapping charges were dropped against the 69-year-old retired service station owner, who apologized in court Monday as he stood before the families and about a dozen members of the El Segundo police department.
He was immediately sentenced to two consecutive life terms.
"I feel like I am dreaming," Mason said. "It makes no sense. It's contrary to everything I believe. At no other time in my life have I intentionally harmed anyone. I don't know why I did this."
"Please forgive me," he said. "Do not be bitter."
The Columbia, S.C., man had been living quietly with his wife of 40 years, giving her and their neighbors no indication that he had a criminal past. He was arrested in January.
No members of Mason's family attended Monday's hearing, and his lawyer in Columbia, Chris Mills, said they would not comment.
"From our perspective, he's acknowledged his responsibility. He's apologized in court," Mill told The State newspaper in Columbia. "Now it's time for everybody to move on with their lives."
Neighbors were shocked.
"We want to think he wasn't guilty because of the way he carried himself," Bill Wilson told the newspaper. Added longtime friend Jimmy Woods: "This is just flabbergasting."
Deputy District Attorney Darren Levine said his office would not oppose a request by Mason to serve his sentence in South Carolina near his wife and family.
El Segundo officers Milton Curtis, 25, and Richard Phillips, 28, were shot to death on July 22, 1957, by a man they had pulled over for running a red light.
About 90 minutes before the shooting, California authorities say Mason robbed two 15-year-old girls and their dates. One of their girls was sexually assaulted.
The case languished until police got a tip last September that someone had bragged about the killings. The lead was false but it prompted a review in which the FBI's fingerprint database matched prints from the stolen car to Mason, who had spent time in jail for a 1956 burglary conviction in South Carolina.
In court Monday, the children of the slain men spoke of the grief their families endured without their fathers.
"Your cowardly act shattered our lives," said Carolyn Phillips, who spoke of her mother's struggle to raise three children. "We cannot and will not forgive you."
Outside court, Keith Curtis, the son of the other slain officer, said he was not moved by Mason's courtroom apology.
"Mr. Mason is sorry now and we heard his apology speech," he said. "He wasn't sorry for 45 years and the only reason we're hearing that apology now is because he got caught."
By Linda Deutsch