A man who pleaded guilty and testified against his co-defendants on trial for the killing retired Chicago firefighter Dwain Williams has been sentenced to 30 years in prison.

Dwain Johnson, 20, was sentenced Friday morning by Cook County Judge Timothy Joyce, just days after his testimony helped secure the convictions of Devin Barron and Jaylen Saulsberry.

The judge said the sentence would have been longer if Johnson had not pleaded guilty and was convicted, and he noted the “generosity” of Williams’ family in supporting the deal reached by prosecutors.

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Williams’ family said they were consulted by prosecutors and approved of the decision to offer a deal to Johnson after three separate juries were empaneled to hear each defendant’s case.

In statements during the hearing Friday, Williams’ family said they were still dealing with their grief over his loss and told Johnson nothing could change that.

“‘He was a hero and will always be remembered,” one woman said of Williams before looking at Johnson and asking, “Will you always be remembered as a vicious murderer?”

Johnson declined to address the court.

On Tuesday, Johnson took the stand identified Barron and Saulsberry in surveillance video that showed them getting out of a stolen Ford sedan and confronting Williams at gunpoint as they tried to steal the retired fire lieutenant’s Jeep Grand Cherokee Trackhawk SUV on Dec. 3, 2020.

Williams, 65, was confronted by the carjacking crew as he returned to his SUV after making a purchase at Let’s Get Poppin’ at 11758 S. Western Ave. Williams drew his own gun and exchanged fire with Barron and Saulsberry, but was shot during the encounter and later died.

Johnson wasn’t accused of being one of the gunman but testified that he took part in the carjacking attempt that led to Williams’ shooting.

Both Barron and Saulsberry were found guilty of murder on Wednesday.

Johnson’s deal saw the murder charge against him reduced to aggravated battery with a firearm, and he accepted that prosecutors would recommend the maximum sentence against him.

Johnson was given credit for a little more than five years off his final sentence for time spent in custody awaiting trial and for programs he enrolled in while in custody. He will be required to serve at least 85% of his sentence.

Barron and Saulsberry are awaiting sentencing. They are expected back in court Aug. 3 for post-trial motions.