Born in Belfast, Neill played as a youth for Bangor, before moving in 1959 to Arsenal. He made his debut in December 1960 against Sheffield Wednesday, becoming an accomplished centre half (but also playing at full back). He became a regular in the Arsenal side of the 1960s, as well as the Northern Ireland national side, and went on to captain both club and country. In all he played 255 times for Arsenal and 59 times for Northern Ireland.

Towards the end of the 1960s, Neill became affected by injuries and a bout of jaundice, missing the 1969 League Cup final (which Arsenal lost). Although still only 28, he was signed by Hull City in July 1970 as player-manager, one of the youngest ever managers in the history of the game; he later became player-manager of his country as well. Neill retired from playing in 1973, and left Hull a year later to succeed Bill Nicholson as manager of Arsenal's fiercest rivals, Tottenham Hotspur. He managed Spurs for two seasons, nearly getting the club relegated in the process.

Despite his less than sterling record at Spurs, Neill was recruited by the Arsenal board to replace Bertie Mee in 1976, to become the youngest manager in the club's history. With new signings like Malcolm Macdonald and Pat Jennings, the club enjoyed a minor revival, reaching a trio of FA Cup finals between 1978 and 1980, though only winning the middle one of the three, and the 1980 final of the Cup Winners' Cup (which Arsenal lost on penalties to Valencia). However, Arsenal's success in the cups could not be matched in the league, and the departures of star players such as Liam Brady and Frank Stapleton only made things worse. A series of embarrassing cup defeats in the early 1980s made things worse; a League Cup loss at home to Walsall proved to be the final straw, and he was sacked in December 1983.

Neill retired from football, and has since opened sports bars in Hendon and Holborn, central London.