The final autopsy report was issued by medical examiners in San Bernardino County two months after King, 47, was found submerged at the deep end of a swimming pool at his home in Rialto, a suburb about 80 km east of Los Angeles
The death was ruled an accidental drowning, and listed "alcohol and multiple drug toxicity" as contributing causes, Rialto police Captain Randy De Anda told Reuters.
He said coroners found cocaine, PCP and marijuana in King's body.
"Our investigation is now concluded," De Anda said. "The conclusions that our investigation revealed have been reinforced by the autopsy and toxicology report."
The findings of the autopsy and other postmortem studies performed by medical examiners "indicated he was in a state of drug- and alcohol-induced delirium at the time of this terminal event," he said, quoting from the report.
The coroners concluded that King fell or jumped into his backyard pool, he added.
He said medical examiners listed abrasions and other signs of "minor trauma" consistent with someone drowning or being pulled out of a swimming pool.
Police were called to King's home on June 17 by his fiance, Cynthia Kelly, who told authorities she was awakened by a noise to find him at the bottom of the pool, but was helpless to go to his aid because she did not swim.
Police retrieved his lifeless body from the water, and efforts by officers and paramedics to resuscitate him proved futile.
His death left many wondering how King, who was by all accounts an avid swimmer, could have drowned in his own pool. The autopsy's findings of drug and alcohol use seemed to answer that mystery.
King's death came two months after the 20th anniversary of Los Angeles riots triggered by the acquittal of four white police officers prosecuted for beating King, who was black, in a confrontation a year earlier caught on videotape and widely replayed.
During the unrest, which left over 50 people dead and caused more than $1 billion in property damage, King famously appealed for calm in a televised appearance in which he asked rhetorically, "Can we all get along?"
King, who long struggled with drug and alcohol abuse, financial difficulties and legal problems, had this year published a memoir entitled: "The Riot Within: My Journey from Rebellion to Redemption."