After the judge agreed to resentence them on Tuesday afternoon, the brothers delivered an emotional statement to the court.
They went through grim details of the brutal killings and their decision to reload their shotguns and keep shooting their parents at point-blank range in the living room of their home. The siblings were aged 18 and 21 at the time.
"I had to stop being selfish and immature to really understand what my parents went though in those last moments," Erik Menendez, 54, told the court.
He described the "shock, confusion and betrayal" they must have felt seeing their sons holding guns and opening fire.
Both apologised for their actions and talked about their hopes of working with sex abuse victims and helping those incarcerated if they were given a second chance outside prison.
The voice of Lyle Menendez, 57, cracked as he talked about the impact of his "unfathomable" actions on their relatives.
"I lied to you and forced you into a spotlight of public humiliation," he said to his family.
Los Angeles County District Attorney Nathan Hochman said the decision to resentence the pair was "monumental" and carried "significant implications for the families involved".
Much of the hearing centred on what the brothers have been doing in prison for the last 30 years.
Family members and those who worked with them in prison detailed the educational courses they completed and programmes they created to improve the lives of inmates, including a hospice initiative for the elderly and sick.
Judge Jesic called the brothers' work while in prison "remarkable", but noted that their 1996 sentences to life without parole were justified at the time.
He said under the guidelines, they were eligible for resentencing, issuing his new sentence of 50 years to life. The brothers have been held in custody since 1990.