Mahoney was born in Cardiff and grew up in Manchester as his father played Rugby league with Oldham R.L.F.C.. Mahoney began playing non-league football with Ashton United before signing for Crewe Alexandra in March 1966. With Liverpool showing a big interest in Mahoney, Crewe's former Stoke player Johnny King recommended him to Stoke manager Tony Waddington and he duly took his advice and signed him for £19,500 in March 1967. A rough diamond at first, Mahoney regularly lost possession by 'over-elaborate dribbling'. Waddington taught him the art of pass and move and he became a quality midfielder sitting just in front of his defence his task was to win the ball back and play in his wingers or forwards. Due to competition for places he was subject of a £50,000 offer from Millwall in February 1972 but rejected the chance to move to London. His patience was rewarded with a substitute appearance in the 1972 Football League Cup Final as Stoke beat Chelsea 2–1.