Duncan's managerial career started as Player/Manager at Scunthorpe United and included two spells at Chesterfield (1983-1987 and 1993-2000), during which time the club achieved two promotions and won through to an FA Cup Semi-Final in 1995. During his time at Chesterfield he managed the club for a total of 508 games, losing only 32% of them in total.
In addition, John had a short spell at Hartlepool United and three seasons at Ipswich Town.
A qualified teacher, John started his career at his hometown club Dundee where he won a League Cup Winners medal, before transferring to Tottenham Hotspur in 1974 for a fee of £160,000. In a relatively unsuccessful Spurs side, going through a transitional period, John had an excellent scoring record getting 62 goals in 118 appearances, and was a key member of the Spurs team that regained their place in the old First Division in 1977/78.
He was later transferred to Derby County, before starting out on his managerial career.