Jonathan Mishne’s first electric guitar was hand-crafted by a fellow inmate at Thomaston State Prison. To pass the long, monotonous days during his internment, Mishne would play his guitar and sometimes jam with other musicians. When his parents gave him a guitar, he stuffed a pillow into the body of it at night so he could play without disturbing cell mates. It wasn’t clear to Mishne then that his music would be his salvation, his ticket out of jail.

“If I didn’t have the luck, the guitar and family support, I would have been derailed even more,” said Mishne, who had been given a five-year sentence for drug-related crimes. “I did my time, and it was really hard to do. You miss your family and your life, and you don’t know where you’re ever gonna go.”

This weekend, eight years after his release, Mishne will go back to Thomaston, this time to perform a concert in the prison yard with his Boston-based string quartet, the Boccherini Ensemble.

While there, he will probably pass by Cell Block K7, where he spent three years and nine months playing his guitar. But, Sunday, when the group finishes playing Bach and Shubert and Jimi Hendrix and Led Zeppelin, Mishne will walk out of the security doors for the first time as a free man.

When Mishne first left the prison in 1982, he had been granted a school release, one that allowed him to study music at the University of Maine at Augusta while living at a pre-release center in Hallowell. Another inmate had told Mishne about the music program at UMA and had helped him prepare for the admission requirements. Mishne, who had studied piano as a child, wasn’t sure he was playing the songs correctly, but he sometimes worked on them 14 hours a day.

After writing countless letters requesting admission to the program, Mishne received a visit from a UMA adviser who escorted him to the university for an audition. In Augusta, music professor Tom Hoffmann listened to Mishne play and spontaneously picked up his own guitar to play along. When they finished, Hoffman said it was a waste for Mishne to be in Thomaston and that the university would advocate for his release.

“If I had gotten out with $50 and without a dream, I may have ended up in the same jam I had been in,” reflected Mishne.

In 1979, at the age of 21, Mishne was convicted of armed robbery, extortion and kidnapping. In search of narcotics, he and a friend had used unloaded guns to force their way into a doctor’s office where they demanded to be given drugs. After the doctor convinced them there were no such substances in the office, Mishne drove the doctor to a pharmacy where Demerol was purchased. Then Mishne shot up in the back seat of the car. At that moment, as his withdrawal symptoms subsided, he realized the trouble he was in.

Mishne drove the doctor back to the office and asked him not to phone the police. Three days later, Mishne was arrested.

Although the circumstances were undesirable, the time in jail allowed Mishne to devote himself entirely to his music and to consider the choices and mistakes he had made in life. These reflections surfaced later in musical compositions such as “Chester,” a song Mishne wrote about a fellow inmate who hung himself. Mishne did not write a part for his guitar in this number, which he describes as both disturbing and peaceful. “I don’t want to play in that piece,” he said.

In performance, “Chester” is always followed by “Box A,” a composition named after the address at the state prison. When Mishne introduced the piece to his musical ensemble, he said it was to be performed “as painfully as possible” so listeners could hear the frustration and anger of life behind bars. Mishne’s guitar resounds in this number.

“I want to go to Thomaston to give something back,” said Mishne, who graduated in 1988 with a bachelor’s degree from the New England Conservatory of Music.

A year ago, when Mishne founded the Boccherini Ensemble, he wasn’t willing to talk about his past for fear that it would jeopardize his success. He didn’t want to risk losing the second chance he had worked so hard to get.

But Mishne and his young group, violinist Zoia Bologovsky of the Bangor Symphony Orchestra and The Julliard School, violist Rohan Gregory of the Portland Symphony Orchestra and Oberlin Conservatory of Music, and cellist Charae Baerg of the Cape Cod Symphony and the New England Conservatory, are swiftly establishing a reputation as vanguards of a contemporary and accessible approach to chamber music.

This year, the group was chosen from among 1,400 applicants to perform at the European International Festival in Geneva. They also play regularly at jazz and rock clubs in New York City and Boston.

“We are trying to break down the barriers between classical and rock,” said Mishne, whose group includes a fifth member — percussionist Daniel Foote — for the rock numbers.

The ensemble will play rock, jazz, bluegrass and classical music in the prison yard on Sunday. And although Mishne initially thought this gig would be a pressure-free performance, as the date nears, he wonders how he will feel when he enters those doors once again. He hopes the inmates at Thomaston will enjoy the performance, but knows he’ll be thinking about the price he paid for a shot-glass full of Demerol.

“I told the group that there are people in there with heart, and it’s good to play for people with heart, people who won’t get to see a symphony or wear a tux,” he said. “I want my friends there to have a nice day and just for a minute forget that they’re doing time.”