Taha Yassin Ramadan, Iraq's former vice-president, has been hanged, an official with the prime minister's office said.
The Iraqi government was reported on Monday to have requested that US officials hand over Ramadan into Iraqi custody before his execution.
Badie Aref, a lawyer in the case, said the US military "allowed Ramadan to call his family" on Monday.
"He was very calm and composed. He asked his family and friends to pray for him and said that he was not afraid of death."
Ramadan was convicted over his role in the killing of 148 Shia Iraqi citizens from the town of Dujail in the 1980s. An appeal court upheld the death sentence against him last week.
The killings took place in response to a failed assassination attempt on Saddam Hussein, the former president.
Ramadan will be the third aide of Saddam to be hanged for crimes against humanity.
Saddam himself was hanged for the Dujail killings on December 30 while Barzan Ibrahim al-Tikriti and Awad Ahmed al-Bandar were executed on January 15.