A federal jury had found Slatten guilty of first-degree murder in December.
Slatten was convicted for killing Ahmed Haithem Ahmed Al Rubia'y, 19, an aspiring doctor who was one of more than a dozen civilians killed by Blackwater guards in Baghdad's Nisour Square on 16 September 2007.
Slatten, a sniper, is said to have fired the first shots.
Baghdad massacre: US court finds ex-Blackwater mercenary guilty of murder
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Before the sentencing on Wednesday, friends and relatives requested leniency for Slatten, AP news agency reported.
The defence had argued that Slatten and other Blackwater contractors opened fire because they thought they saw a potential suicide car bomber moving towards a diplomatic convoy they were escorting.
The Blackwater guards opened fire in the bustling square with sniper rifles, machine guns and grenade launchers, leaving at least 14 civilians dead and at least 18 wounded, AFP news agency reported.
The Iraqi government says the death toll was higher in the incident, which is sometimes referred to as the Nisour Square massacre.
Slatten's conviction in December was the second time a federal jury in Washington found him guilty of first-degree murder.
A previous conviction in 2014 was overturned on appeal, and a second trial during the summer of 2018 ended in a hung jury.
Three other Blackwater contractors involved in the Baghdad massacre - Dustin Heard, Evan Liberty and Paul Slough - were convicted in 2014 of voluntary manslaughter and using a machine gun to carry out a violent crime.
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A fifth Blackwater contractor, Jeremy Ridgeway, had previously pleaded guilty to manslaughter and cooperated with prosecutors.
In May, the New York Times reported that two US officials said President Donald Trump was considering a presidential pardon for Slatten and several others.
Earlier this year, the US president pardoned an American soldier who was convicted of murdering an Iraqi detainee in an unrelated incident in 2008.
Rights advocates decried that decision as an endorsement of extrajudicial executions.
Founded by former Navy SEAL Erik Prince, Blackwater was sold and now operates as Virginia-based Academi.
Blackwater has been at the heart of widespread criticism over the use of private contractors in Iraq following the US invasion in 2003.
The Baghdad shooting, in particular, stood out for its brutality and sparked debate over the actions of mercenaries employed in support of the US army in the country.