Despite being a Stoke City fan, Machin started his playing career at nearbyPort Vale, signing professional forms in July 1962. He made the odd appearance from October 1962 and 1964, after which point he started to appear rather more frequently. He was not a favourite with Stanley Matthews or trainer Lol Hamlett as he had a habit of talking back to the pair, so he put in a transfer request in 1966. In all competitions he made 32 appearances, scoring 6 goals, being utilised mostly as an inside-forward.
After retiring as a player, Norwich then invited him to join their coaching set-up. He worked as youth team and reserve team coach, and then was promoted to chief coach before being appointed as assistant to manager Ken Brown. The partnership finished in May 1987, when he accepted an offer to manage Manchester City.
After two seasons in charge, he got the club promoted to Division One with a young and promising squad. In the first season in the top-flight, his team beat local rivalsManchester United by 5–1 on 23 September 1989, in what Alex Ferguson described as the lowest point of his career. Despite the victory, two months later, on 27 November, Machin was sacked by chairman Peter Swales as the club was bottom of the division. He became Barnsley manager on 29 December but resigned on 5 May 1993, as he was disillusioned with the club policy of selling their best players to make ends meet.
Machin then worked as a scout for West Ham United, Tottenham Hotspur and Liverpool before he was appointed manager of Bournemouth in September 1994. In his first season at the helm he managed to keep the club in the Second Division despite a start with seven consecutive defeats and a serious financial crisis, this feat later became known as "The Great Escape". In 1998, Bournemouth lost to Grimsby Town in the Auto Windscreens Shield final in their first ever Wembley appearance. In August 2000, he became director of football role and later retired on 29 August 2002, having had his testimonial match the previous month. It was in his testimonial – a 3–2 victory for Manchester United, that United's £29.3 million signing Rio Ferdinand made his debut. In October 2002 he was linked to the vacant management position at Swindon Town.