Thomas told jurors that Floyd, 46, died from cardiopulmonary arrest complicated by the way Chauvin restrained him and compressed his neck.
She added: “The primary mechanism of death is asphyxia, or low oxygen.”
The court in Minneapolis has been shown a nine and a half minute clip of the officer appearing to be kneeling on Floyd’s neck throughout. The defence has claimed that pressure on the neck was not exerted for the full time.
Thomas, who had worked as a pathologist at the county medical examiners’ office, said: “This is a death where both the heart and lungs stopped working. The point is, it’s due to law enforcement subdual, restraint and compression.
“The activities of the law enforcement officers resulted in Mr Floyd’s death.”
Thomas said she reached her conclusion primarily from the video, which showed Floyd “in a position where he was unable to adequately breathe.”
The autopsy itself ruled out heart attack, aneurysm, COVID-19 and other factors, and Thomas said it was not a drug overdose death, either. However, it found fentanyl and methamphetamine in his system.
Floyd’s death certificate listed other contributing conditions: narrowed arteries, high blood pressure, fentanyl intoxication and recent methamphetamine use.
But Thomas said that they did not directly cause his death and that such factors are commonly included on death certificates to inform public health officials.
Floyd was arrested outside a neighborhood convenience store after being accused of trying to pass a counterfeit $20 bill.
Bystander video of Floyd crying that he couldn’t breathe as onlookers yelled at the white officer to get off him sparked protests and scattered violence around the US.
Thomas’ testimony came a day after other medical experts also said Floyd died of a lack of oxygen.
Prosecution witness Dr Martin Tobin, a lung and critical care specialist, told the court: “A healthy person subjected to what Mr. Floyd was subjected to would have died.”
Tobin said the lack of oxygen resulted in brain damage and caused Floyd’s heart to stop.
Chauvin, 45, is charged with murder and manslaughter in Floyd’s death on May 25 after kneeling on him for what prosecutors say was nine and a half minutes.
His defence lawyer Eric Nelson has argued that officer did what he was trained to do and that Floyd’s death was caused by illegal drugs and underlying medical problems, including high blood pressure and heart disease.
The trial of the former policeman, who denies the charges, continues.