He suffered a cardiac arrest while being restrained by Minneapolis police officers on 25 May, the report found.
It listed Mr Floyd's cause of death as "cardiopulmonary arrest complicating law enforcement subdual, restraint, and neck compression".
The examination also showed he had heart disease and recent drug use.
The findings of the official post-mortem were released shortly after those of a private examination carried out by medical examiners hired by the Floyd family.
That report said Floyd died from asphyxia (lack of oxygen) due to compression on his neck and back. It also found the death was a homicide, a statement from the family's legal team said.
A video showing a white police officer continuing to kneel on George Floyd's neck even after he pleaded he could not breathe sparked outrage when it emerged a week ago.
It has led to six consecutive days of protests around the United States and a level of civil unrest not seen this widely across the country in decades.