Leo, then just 17, travelled up to 200 miles from south London to areas like Dartmoor and was stabbed six times by rivals.
He is one of the inspirations behind County Lines, a gritty new British film examining the £1billion-a-year trade, from director Henry Blake.
Blake has told the Evening Standard how he drew on his 11-year experience as a youth worker at a pupil referral unit in east London for County Lines.
In the film actor Conrad Khan plays 14-year-old Tyler Hughes, who is excluded from school and exploited by dealer Simon (Harris Dickinson) to become a train-bound drugs courier.
At school, Leo was predicted to achieve all B grades at GCSEs having overcome dyspraxia, a condition that causes children to perform less well than expected for their age.
After being bullied by “older men” at 15, Leo sought the support of “gangsters who drove expensive cars” in his area.
He was forbidden from selling drugs at first. But they encouraged him to fight knife battles to protect the gang’s territory and earn a reputation.
Leo told the Standard: “I just got on this money bandwagon and that’s when everything changed – even friendships.
It doesn’t matter what background you’re from. I’ve seen people whose parents are middle class and they come from wicked households. They’ve given that child everything - money and good morals - and that young person is still moving drugs
“Everyone started attacking, having conflict and killing each other.”
His brothers “took him under their wing” and the family started moving drugs from the capital to Lincolnshire, Felixstowe, Dartmoor and South Wales.
Leo added: “In two years, I’d gone from being a big Harry Potter fan obsessed with reading about the world of Hogwarts to racing down the M4 at 100mph in a rented Audi with hundreds of £10 deals.”
Leo said he had been ambushed several times on the street and knifed in the head, leg and twice in his back.
The first attack was nearly fatal but he discharged himself from hospital against doctors’ orders.
He admitted: “The people who stabbed me were people I had issues with, it’s not like it was random.
“I did stuff to them and it was their time to catch and pull up on me.”
County lines is the movement of drugs from large cities like London to smaller towns and counties.
Dealers usually turn up in an area with burner mobiles capable of sending 500 messages at a time to potential customers.
Leo claimed within just three weeks, he and his brothers managed to turn each line out of London into a £1,800-a-day operation.
They continued buying drug supplies in bigger amounts “because you never go down”, which meant being away from home for a year at a time, fleetingly seeing their mother for 10 minutes.
Leo was arrested by police several times but insists he never went to jail.
He reflected: “I was 22 and I walked into my mother’s house and she was crying. She was asking God why she failed as a parent. That really hit home and I just sat down and thought, ‘What the hell have I been doing?’
“I could count on one hand how many times I had seen her since the age of 17, probably just four and two of those were for 10 minutes.
“I was in and out of the house, then I’m gone for a year.
“Everyone out there doing this craziness knows it’s wrong. I’d lost myself and seeing her crying and praying for me and my brothers, I knew I had to change.”
When Leo told his partners he was walking away from the county lines operation, he was mocked and arguments erupted.
He added: “I told the boys, ‘I don’t want this no more’. I wanted to get a job and go legit. Their response was like, ‘You’re talking s***’
“Three months later, they pulled up on me in the street in a flash car and said, ‘Do you want back in?
“But I replied, ‘Nah, I’m good’.
I just got on this money bandwagon and that’s when everything changed – even friendships.
“It doesn’t matter what background you’re from. I’ve seen people whose parents are middle class and they come from wicked households. They’ve given that child everything - money and good morals - and that young person is still moving drugs.”
Leo returned to education and retook exams. He is glad that he got out because two ex-friends have been sectioned at high-security Broadmoor psychiatric hospital.
He said: “I despise drugs and violence that comes with it. Cocaine, heroin, ecstasy and cannabis destroy lives and families.
“One of the biggest diseases facing young people today is trying to please other people.
“Kids have been groomed and brainwashed by dealers. They worship money and can’t think as far as putting it in their own bank account, they just want it in their pocket.
“You’ve got youngsters arriving in prison dressing sharp in Tom Ford T-shirts, Versace jeans and Audemars Piguet watches because they don’t want to be seen as weak.
“They don’t know about building assets and owning your own home. As long as there’s that mentality, this war will never end.
“I knew I had to change because it was either prison, serious mental health or death.”
Met Deputy Assistant Commissioner Graham McNulty welcomed the County Lines film, saying it will raise awareness of the signs to look out for to protect vulnerable children.
Mr McNulty, the UK’s national lead on county lines, said Operation Orochi had closed down 200 London drug networks and arrested 345 line-holders since November 2019.
Around 650 charges have been brought for supply and modern slavery.
“As I speak to you, there are over 100 live operations in London to bring down county lines. It’s a real focus for us because of the violence that comes from it.”
British Transport Police’s national crackdown on county lines said of 805 arrests, the youngest was just 13.
Officers have seized more than 141 dangerous weapons, £263,000 in cash, 517 phones and 401 quantities of drugs.
*Leo’s name has been changed to protect his identity.
County Lines will be released in cinemas and on BFI Player and Curzon Home Cinema on December 4.