Ferris became involved with Thompson's crime empire aged 19, when he became an enforcer collecting debts on behalf of Thompson, and was linked to stabbings, slashings, blindings and knee-cappings. A year later Ferris was arrested following an incident where shots were fired at Devlin and three of his relatives while they travelling home from a night at a pub, with Devlin's father-in-law sustaining a bullet wound to his thigh. The three relatives failed to identify Ferris at an identity parade, but Devlin picked him out as the man who fired the shots. Ferris was charged with four counts of attempted murder, and was remanded to Longriggend. At his trial several months later he was acquitted on all four counts with a not proven verdict, a type of acquittal described by Sir Walter Scott as "that bastard verdict".
Now aged 21, Ferris immediately returned to his work as an enforcer for Thompson, and was soon arrested again and charged with possession of offensive weapons after a pickaxe handle and knives were found in his car. While awaiting trial he was involved in a stabbing, and fled to Thompson's holiday home in Rothesay on the Isle of Bute. Within a day of arriving there, he was arrested by armed police and charged with various offences including attempted murder and possession of heroin with intent to supply, and was remanded to HM Prison Barlinnie. The attempted murder charge was dropped a week later and Ferris was found not guilty of the drugs charge, but he received an 18 month sentence for possession of offensive weapons. After being released from prison Ferris stopped working for Thompson and started a company named Cottage Conservatories specialising in double glazing and conservatories, but still remained active in the criminal underworld.
On 18 August 1991 Thompson's son, Arthur Jr (nicknamed "Fat Boy") died after being shot outside his home. Ferris was arrested following the killing, and was charged with murder and remanded to HM Prison Barlinnie. On the day of Thompson Jr's funeral the cortege passed a car containing the bodies of two friends of Ferris, Robert Glover and Joe "Bananas" Hanlon", who were also suspected of involvement in his death, and had been killed by gunshots to the head. At his trial in 1992 Ferris was defended by Donald Findlay, one-time vice-chairman of Rangers Football Club, on the following charges:
- the murder of Arthur Thompson Jr, with help from Glover and Hanlon;
- the attempted murder of Arthur Thompson Snr by repeatedly driving a car at him in May 1990;
- threatening to murder William Gillen, and shooting him in the legs;
- conspiracy to assault John "Jonah" McKenzie on 26 May 1991;
- illegal possession of a firearm;
- supplying heroin, cocaine and ecstasy;
- breach of the Bail Act.
Over 300 witnesses were called to give evidence at a trial which lasted fifty four days and cost £4 million, at the time the longest and most expensive trial in Scottish legal history, and ended in Ferris being acquitted of all charges. After the trial Ferris returned to Glasgow and set up a car dealership named Jagger Autos, and also became a consultant for security firm Premier Security, which had a reported annual turnover of £6.2 million. He also maintained contacts in the underworld, including Paul Massey and Rab Carruthers in the north of England. In 1993 his brother Billy escaped from a prison escort after being allowed temporary release to visit his sick father, and was one of the six most wanted men in Britain until being captured in Blackpool. Later that year, Ferris accused the police of having a vendetta against his family after Billy was refused permission to attend the funeral of their father. In August 1994 Ferris received a £250 fine from a court in Manchester after being charged with possession of crack cocaine. He told the court "I'm no gangster", and claimed he used the drug to alleviate his psoriasis. In 1995 Ferris appeared in a television interview with John McVicar in which he remarked "If anyone was born into crime, it was me. Crime is in my blood", and also claimed he had "always used a weapon of sorts".
Ferris was arrested in London in 1997 following a two year surveillance operation by MI5 and Special Branch. At his trial at the Old Bailey in July 1998 he was sentenced to ten years imprisonment after being convicted of conspiracy to sell or transfer prohibited weapons, conspiracy to deal in firearms and possessing explosives, although the sentence was reduced to seven years at the Court of Appeal in London in May 1999. While in prison Ferris co-authored his biography
The Ferris Conspiracy with Reg McKay, which sold 20,000 copies while he was imprisoned. Ferris was released from Frankland Prison, County Durham in January 2002 pledging to give up his life of crime, and released a second book with McKay, a novel titled
Deadly Divisions, in April 2002. In May of the same year he was sent back to prison for breaching the conditions of his parole after being involved in a knife fight with Tam McGraw, and an alleged connection with a £900,000 shipment of cannabis. Ferris was released again in June 2002, and returned to Scotland.