Demetrius Johnson smiled broadly as he walked out of the county’s main criminal courthouse Friday and headed home for a celebratory bubble bath.

It was a pivotal day for Johnson. The 44-year-old, who went to prison for 12 years as a teenager for murder, became the 20th person to be exonerated based on allegations he was framed by retired Chicago Police Detective Reynaldo Guevara.

Earlier this year, it came to light that an eyewitness account implicating another person in the fatal 1991 Wicker Park shooting had been concealed from prosecutors and defense attorneys.

A Cook County judge vacated Johnson’s conviction in November, leaving it up to the Cook County State’s Attorney’s office to retry the case — although Johnson had served his sentence and couldn’t be sent back to prison — or drop the case altogether.

A prosecutor with the state’s attorney’s office Friday informed Judge Dennis J. Porter that the state’s attorney’s office had decided to drop all charges.

“It’s an out-of-body experience,” Johnson said after the hearing. “I felt like I had a gorilla on my back.”

The move cleared the way for Johnson, who was arrested for the murder at age 15, to apply for a certificate of innocence.

He was released from prison in 2004. The murder conviction, however, has hung over his head, most recently preventing him from passing a background check required to coach his 10-year-old son’s basketball team.

“We were preparing for a re-trial,” Johnson’s attorney Joshua Tepfer said. “We appreciate the state’s attorney making this decision before the holidays.”

Johnson, who lives on the Northwest Side, has four kids and works with churches and community groups to try to prevent gang violence.

His son, Demetrius Johnson Jr., 28, was elated Friday.

“I feel awesome because really for my whole life I thought my dad was guilty of this crime, and the whole time he was innocent and it messed up my life a lot,” he said.