The men confessed to the murder, saying they kidnapped the American for ransom but did not mean to kill him.
An Iranian man and four Iraqis were given life in prison for the 2022 killing of US citizen Stephen Troell in Baghdad.
An Iraqi court sentenced the man on Thursday in a move welcomed by Washington.
“The Iranian man was the mastermind of the crime,” one legal source said. All five involved were arrested in Iraq soon after Troell was killed.
Troell was killed after an attempted abduction last November, police said at the time. He had been living in Baghdad with his family. US media reported he was an English teacher.
Troell was shot dead while driving in Baghdad’s Karada shopping district, an Iraqi interior ministry source said last year.
The men “confessed” to the murder and said they planned to kidnap Troell for ransom but did not mean to kill him, a judicial source told the AFP news agency.
The four Iraqis were not named but court officials said they were members of a militia.
The US State Department welcomed the sentencing. “It is critical that all those responsible for the brutal, premeditated assassination of Mr Troell face justice and accountability,” spokesperson Matthew Miller said in a statement.
While the security situation in Iraq has improved since Baghdad declared victory over the armed group ISIL (ISIS) in 2017, violence still exists.