According to the Los Angeles County Department of Medical Examiner-Coroner, Miller, 26, passed away from "mixed drug toxicity", that included "fentanyl, cocaine and ethanol".
Paramedics found Miller unresponsive in his Los Angeles home on September 7 and declared him dead soon after.
The coroner ruled the death an accident.
More details about rapper Mac Miller's untimely passing have been revealed.
He released his fifth and final album, Swimming, which addressed his struggles with substance abuse, on August 4. Empty bottles of liquor were also found on the property, and a rolled $20 bill in the rapper's pocket with traces of white powder was found in his pocket, the report stated.
Mac Miller, real name Malcom James McCormick, died earlier this year.
Fentanyl, a powerful painkiller that tends to be cheaper and more potent than heroin, has played a major part in the USA's opioid crisis.
The singer Prince had taken fentanyl before his death in 2016.
The night before his death, Miller was reportedly casually hanging out with friends and was watching football. She posted an affectionate video of him on her Instagram page weeks after his death, and released a new song, Thank U, Next, on Saturday that mentioned Miller along with Saturday Night Live star Pete Davidson, her fiance until recently.
On Oct. 31, musicians like Chance the Rapper, John Mayer, Anderson.Paak, Travis Scott and others came together for a Mac Miller tribute concert.