Van Dyke is subject to a minimum sentence of 18 years, prosecutors said in a court document this week.
A jury in October convicted a white police officer, Jason Van Dyke, of second-degree murder for firing 16 bullets into McDonald.
Gaughan did not sentence him for the aggravated battery conviction, explaining second-degree murder was the more serious crime.
This story is part of16 Shots, a podcast about the fatal police shooting of Laquan McDonald, the trial of Officer Jason Van Dyke, and the troubled relationship between African-Americans and the Chicago Police Department.
A judge has acquitted three Chicago police officers of trying to cover up the 2014 shooting of black teenager Laquan McDonald to protect another officer who pulled the trigger. While awaiting an officer to arrive with a Taser, Van Dyke shot McDonald, according to testimony. Van Dyke had claimed that McDonald had been swinging the folding knife in an "aggressive exaggerated manner", a claim that was supported by his fellow officers. "They convicted for the first time, in Chicago history, they convicted an officer in the line of duty regarding a shooting that took somebody's life".
Van Dyke's sister, Heidi Kauffunger, told the court that her brother has been abandoned by family and friends since he was charged.
Vidale Joy said Van Dyke used the slur after pulling him over in 2005 and put a gun to his head.
One witness, Edward Nance, who filed a complaint with the former Independent Police Review Authority and later won a $350,000 civil lawsuit, was so emotionally overwrought on the witness stand, he cried openly and didn't want to look at Van Dyke.
Friday's sentencing in a Chicago courtroom began early Friday morning and lasted well into the evening before the judge handed down the sentence.
Van Dyke's relatives tried to defend and humanise him, saying he's a good father and not racist.
In the Van Dyke case, both the defense and prosecution positions on sentencing reflect the unique nature of the case given that Van Dyke, a longtime public servant, has no previous criminal record.
On Thursday, Cook County Judge Domenica Stephenson cleared former officer Joseph Walsh, former detective David March and officer Thomas Gaffney on charges of obstruction of justice, official misconduct and conspiracy. On Friday afternoon, the jury found Van Dyke guilty of second-degree murder.
McDonald's great-uncle, Rev. Marvin Hunter, expressed frustration with the local criminal justice system and called for a federal investigation.
The courtroom will be packed with activists anxious that Judge Vincent Gaughan will impose a light sentence.
She told the court she had written a school essay about the "harsh realities" of police work, and said officer do not care about race, "they care about your safety".
Stephenson decided the case leaned too heavily on discrepancies between the official police reports and a dashcam video of the incident and failed to prove the officers lied, reports NPR. The release of the video sparked protests and political upheaval in the city.
The case cost the police superintendent his job and was widely seen as the reason the county's top prosecutor was voted out of office a few months later.
The judge also noted several times that the "vantage point" of various officers who witnessed the shooting were "completely different".
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