TMZ has obtained the toxicology report from the L.A. County Coroner's Office.
Mac Miller died of an accidental overdose of fentanyl, cocaine and alcohol, according to the coroner.
The 26-year-old, who rose to fame after topping U.S. charts with his debut album in 2011, was found dead at his California home in September.
"The manner of death was certified as an accident", it added. He was already "blue" when someone called 911. Officials pronounced him dead at the scene.
Following some uncertainty, Mac Miller's cause of death has now been revealed.
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The statement concluded that both the passenger and the driver had broken laws for seriously endangering public safety. The bus had 15 people, including the driver, on board at the time of the accident, which took place on October 28.
An empty bottle of alcohol was found on the nightstand, near Mac's body. and a bottle of prescription pills was found in the bathroom. A $20 bill was found in his right pocket, rolled up with a "white powdery residue" on it.
Miller, whose real name was Malcolm McCormick, was found unresponsive on his bed at his home in the Los Angeles area on September 7 by his assistant, according to the coroner's report. Performers at a tribute concert for him last week included Chance the Rapper, Travis Scott and John Mayer.
Just before his death, Miller had announced a joint tour with Thundercat, to start in October.
The department determined that McCormick's death was accidental - making him the latest in a string of overdose deaths involving the opioid fentanyl.