Sykes experienced alcohol abuse and mental health issues and carried out both petty robberies and violent crime. During a brief period of rehabilitation, he fought ten bouts as a professional boxer between 1978 and 1980. On his release from prison in 1977, having unsuccessfully applied for a professional licence in 1973, he applied again, but the
British Boxing Board of Control (BBBofC) insisted that he wait six months before receiving a licence, probably due to his prison record. He finally made his professional debut in February 1978, beating Keith Steve Johnson via first-round retirement. In his second fight, he challenged Neil Malpass for the BBBofC Central Area heavyweight title, losing via disqualification after he was judged to have deliberately head-butted Malpass in the seventh round. After wins over Tommy Kiely and Neville Meade, he again challenged Malpass for the Central Area title. In July 1978, the fight ended in a draw. In his sixth fight, Sykes knocked American David Wilson unconscious and continued to hit him as he draped over the ropes before the referee managed to pull him away. Wilson suffered a brain haemorrhage, was put on a life support machine, and needed a month in the hospital to recover. In June 1979, he challenged for
John L. Gardner's British and Commonwealth titles at Wembley. Sykes prepared for the fight with three weeks of sparring with
Leon Spinks in
Michigan. Gardner was seven years younger than Sykes (Gardner referred to Sykes after the fight as "an old man"), and this was his thirty-first professional fight; in contrast, Sykes had entered the fight after just eight professional bouts. and also travelled to the United States to stand in as a sparring partner for
Leon Spinks. Sykes was a bodyguard to Alex Steene and sparred the future champion
David Pearce. Paul Sykes said; David "Bomber" Pearce was the toughest and most ferocious fighter he faced during his boxing career based on the sparring that took place at the Waterloo Boxing Gym. Sykes, became good friends with Pearce and the Pearce boxing brothers. Sykes' manager, Tommy Miller, later said, "Paul could have gone right to the top, quite easily .. he impressed everybody", but "he was always in trouble one way or another, he'd always loads of worry on his mind." His professional career ended in March 1980 when Nigerian heavyweight
Ngozika Ekwelum knocked him out in the first round of a fight in
Lagos,
Nigeria. It appeared that Sykes had been billed to fight
Lenny McLean at London's Rainbow Theatre on 20 November 1979, but this fight never happened. Lenny Mclean, in his autobiography, later explained: "A week before the off, Sykes went into a club in Wakefield where he lives, got well pissed and had a ruck with four doormen. He did them all but one of them got lucky and put a cut above his eye that took eight stitches to pull together". Sykes was jailed for five years in 1981 for taking out a contract on a union official from Blackpool. He was the holder of the British amateur squat weightlifting record (deep knee bend, 500 lbs).
Professional results } ==Prison==