He grew up idolising the gangsters in his kampung and led a life of drugs and vice until he was caught for trafficking 61g of heroin in 1988 - an amount enough to sentence four people to death.
Narrowly escaping the gallows, Mr John Tang, 55, served close to 14 years in jail instead. Things took a turn when a childhood friend offered him a job in his current company, furniture firm Zhaplin Group, where he is now general manager.
Hoping to encourage more people to help former offenders like him through employment, Mr Tang is an ambassador for the second edition of the Unlabelled Run, organised by The New Charis Mission (TNCM) charity.
"This run carries a very important message," he said. "It is more than about offering a second chance or forgiveness. It is acceptance, and learning how to walk and work together with ex-offenders."
Mr Tang grew up in the 1970s in a Bukit Timah that was nothing like what it is today. He said: "Gang activities were very 'hot', especially in the kampung areas. I looked up to the gangsters like they were heroes. I wanted to be like them.
"At the age of 14, I joined a local gang... I tried to make a name for myself, a reputation, so I got involved in many fights."
It did not take long for him to get involved in loan-shark activities, extortion, and being a bouncer at gambling dens, eventually starting a drug ring to smuggle and distribute heroin in Singapore - until the law caught up with him in 1988.
With heroin hidden in the engine compartment of his car, Mr Tang, who was then 26, was stopped at the Woodlands Checkpoint and caught red-handed - he later learnt that the Central Narcotics Bureau had been tailing him for weeks before it ambushed him.
Every day, when I went to sleep, I hoped that I would not wake up. Every day, I woke up to face another day of fear. My parents cried when they came to visit me. I was so helpless.
MR JOHN TANG, on his four years spent in remand for heroin smuggling before his case was up for trial.
He then spent four years in remand until his case was up for trial.
"I was living under the fear of death. Every day, when I went to sleep, I hoped that I would not wake up," he said. "Every day, I woke up to face another day of fear. My parents cried when they came to visit me. I was so helpless."
Heartbroken as they were, his parents did not give up on him. His father, who was a fishmonger in Jurong, would visit as often as he could, taking a two-hour bus ride to Changi Prison after a long night's work.
However, after a trial that lasted a few weeks in 1992, Mr Tang was spared the death penalty. His lawyer proved that the car bonnet, where the drugs had been hidden, could be opened without a key, casting doubt on whether Mr Tang had been the sole trafficker. This suggested that someone else could have placed the drugs there without his knowledge.
His second chance woke him up.
In his 131/2 years behind bars, he passed his N-, O- and A-level exams, and later earned a group diploma in management accounting as well.
"Because of the trust and support from my family and friends, today I am a businessman who travels abroad for work," he said.
On July 8, Mr Tang and other former residents of TNCM's halfway house will take part in the second edition of the Unlabelled Run in Punggol, hoping to change any impression that former offenders are incorrigible or a menace to society. The first edition of the run took place last year.
Although about 26.5 per cent of former inmates re-offended within two years of their release in 2014, the Singapore Prison Service has said that those who are rehabilitated through employment are less likely to go back to their old ways.
Meanwhile, the number of employers working with the Singapore Corporation of Rehabilitative Enterprises to hire former inmates grew from 4,745 in 2015 to 5,093 last year.
Mr Brian Tan, 33, also a former resident of TNCM's halfway house, said: "With the Unlabelled Run managed primarily by ex-offenders, I hope this lets the public know that we can, and have changed."
Mr Tan joined a gang at 16 and took drugs two years later. His addiction led him to rack up about $60,000 in debts. His parents paid these off for him over the years with their savings, but it strained their ties. "For years, all I had been thinking about was finding money for drugs, and to spend," he said.
Things changed when he learnt the stories of other former offenders after his first visit to TNCM's halfway house. "I told myself, 'I think I have had enough'. If they can have a new life, I want one too," said Mr Tan, who has since reconciled with his parents. "Lives impact lives, and I hope that our stories can do the same for others."