The coroner ruled that the overdose caused by "mixed drug toxicity" was accidental.
"The manner of death was certified as an accident", it added.
Law enforcement sources say it was clear to paramedics that Mac was dead long before they arrived.
In September, Miller, 26, was found unresponsive in his home in Studio City, California. The assistant described him as already being "blue" when he phoned 911.
The coroner classified the overdose as accidental, TMZ reported.
Miller also reportedly had a quarter-inch abrasion on the bridge of his nose with blood coming out of one nostril.
Video shows woman attacking bus driver after missing stop, causing deadly crash
When the 43-year-old driver, also identified by his last name, Ran, refused, Liu hit him over he head with her cell phone. The female passenger started quarrelling with the driver after she missed her stop, according to Wanzhou district police.
An empty bottle of alcohol was found on a nightstand near his bed.
Earlier this year, following his split from singer Ariana Grande, Miller, whose given name was Malcolm McCormick, was arrested for DUI and hit-and-run.
It has been confirmed that the popular artist died from a drug overdose. The list of pills recovered in the home include Xanax, oxycodone, hydrocodone, and generic Adderall. A $20 bill was found in his right pocket, rolled up with a "white powdery residue" on it.
Friends of the rapper and fellow performers, including SZA, Travis Scott, Ty Dolla $ign and John Mayer, hosted an all-star concert benefitting the Mac Miller Circles Fund in Los Angeles last week.
Just before his death, Miller had announced a joint tour with Thundercat, to start in October.