When 26-year-old artist John Costi, from Finchley, graduated from Central Saint Martins with a first class honours degree in fine art last year, he felt the same way as all his fellow graduates – happy, relieved and incredibly proud.

But what made John slightly different from the other students was the fact that, just three years earlier, he had been in prison for five counts of armed robbery.

John’s graduation ceremony marked the culmination of his transformation from ‘troubled youth’ and convicted offender to contributing member of society and artist – and won him a place in the forthcoming The Catlin Guide 2014: New Artists in the UK, a guide to the 40 most promising new graduate artists in the UK, as recommended by curators, art writers, collectors, gallerists and course tutors.

“It’s good,“ John says, sounding bemused but also pleased. “It’s a very important publication so it’s kind of weird, it’s great that I’m in it. It’s recognition – it’s nice to know that someone else thinks that what I’m doing isn’t a waste of time!“

John was brought up in Finchley by his parents John and Bridget and “didn’t really get on with school“. He attended Finchley Catholic High School for a year and a half and then Compton School for a year before being expelled for “bad behaviour“ – and spent most of his time out on the streets.

“You find yourself in certain situations when that’s the way you live and, yeah, the offending just kind of happened.“

John was sentenced to two and a half years in Feltham Young Offenders Institution for five counts of armed robbery, committed in 2006 and 2007 around Finchley and north London.

“It was caused by personal things that happened in my life that weren’t great that closed the world off for me. It’s a bad attitude, basically. I could make excuses for it and say that tough things happen in life, but that’s just a bit of a sob story, isn’t it? You just have to accept it.

“I look back at that person and it’s a stranger, it’s not me.“

While he was in Feltham, John signed up for an art therapy programme because he had always liked painting and drawing as a child – and had been a prolific graffiti artist all over London – and the tutor asked him to produce six paintings for the refurbishment of the institution’s healthcare wing.

The project was a success and during the final six months of his sentence he was granted day release from an open prison in south London and enrolled on a one-year fine art HNC course at Kensington and Chelsea College, which he passed with distinction.

During the course, John started making artwork using letters he’d received from inmates – revealing to the other students that he’d been in prison.

“There was intrigue and some mistrust,“ he admits. “It was a mixed bag of reactions – some people wanted to talk about it, some people wanted to give me a wide berth.“

But John persevered and, following his full release, was accepted on to the BA fine art course at Central Saint Martins.

His performance piece at last summer’s degree show – which used objects he’d found, recordings and, again, letters from prison inmates to tell his story – played an important part in securing him a place in The Catlin Guide.

The artwork that John produces he describes as “a directed conversation“ and he believes that objects have their own special meaning, and sees himself not as an art-maker or craftsperson but more as an ‘assistant’ or editor, and produces the likes of installations, displays and films.

He now works for the Koestler Trust, which provides a mentoring scheme that John took part in when he was first released from prison, is a member of two art collectives and an art partnership, and runs poetry events in Kings Cross, where he moved a few months ago.

“Art saved my life,“ John says simply.

“If I hadn’t had access to that art therapy course, I probably would have ended up reoffending. It’s a bit cheesy but it keeps me sane.“