Hacker Barnaby Jack, who died in San Francisco in July, overdosed on a mix of heroin, cocaine and other drugs.
Mr Jack, who was born in New Zealand, was found dead in his bed.
San Francisco's medical examiner said an autopsy has revealed that "acute mixed drug intoxication" led to his death.
The BBC reports Mr Jack rose to fame after a 2010 demonstration, in which he hacked a cash machine, making it give out money.
The technique was dubbed "Jackpotting".
He was also an expert in the weaknesses that could be found in medical technology.