Slager, who is white, was caught on a witness’ mobile camera chasing Mr Scott after a 2015 traffic stop and then shooting him in the back as he was running away.
That video footage taken by Feiden Santana proved crucial to the case.
Slager’s first state trial had ended in a deadlocked jury and mistrial last year. In May, he pleaded guilty to using excessive force and violating Mr Scott’s federal civil rights.
His plea resolved the pending state charges and ended the federal case against him.
US District Court Judge David Norton ruled the death was second-degree murder. He also found Slager guilty of obstruction of justice for lying to investigators.
Slager had pulled over Mr Scott, the father of four, for a broken rear light in April 2015.
In the initial state trial, the former police officer said that during the ensuing struggle Mr Scott attempted to take his taser.
However, Mr Scott had never reached for the weapon.
He shot at Mr Scott eight times, hitting him with five bullets. His lawyers attempted to argue his actions were in the heat of the moment.
Slager, fired from the North Charleston police department in the wake of the murder, is one of the few police officers in recent years in the US to receive prison time for an on-duty shooting.
Both men’s families were in attendance at the tense trial in Charleston, South Carolina.
US District Court Judge David Norton said: "What’s just for the Scott family is not necessarily just for the Slager family, and what’s just for the Slager family is not necessarily just for the Scott family...It’s a zero-sum game."
"Everyone recognises that this was a tragedy," he added.
"I wish this never would have happened. I wish I could go back to the day and change the events, but I can’t." said Slager, during his sentencing hearing on 7 December.
At one point during the sentencing trial, Mr Scott’s mother and several other family members said they forgave the killer.
Though Mr Scott’s brother Rodney said afterwards at a news conference that the family was "thankful...the justice system that worked on our behalf”, he said they feel "a lot of work" is still left to be done regarding relations between the police and minorities in the US.