New Celtic manager, Billy McNeill, swooped for Provan in September 1978. The transfer fee of £120,000 was at the time a record in Scottish football. Provan quickly established himself in McNeill’s revamped Celtic side, scoring his first goal against Hibernian at Easter Road on November 18, 1978. In May 1979 Provan clinched his first major winner’s medal as Celtic defeated Rangers 4-2 in the famous ’10 men win the league’ game. That win saw Celtic crowned Scottish Premier Division champions. The following year Provan was voted SPFA Player of the Year by his fellow professionals. Provan was proving to be a huge success at Celtic and his distinctive long curly hair, jersey hanging over his shorts and socks down at his ankles made him – visually alone – a standout figure on the field of play in Scotland.
Provan went on to win a further three League championships with Celtic (1980–81, 1981–82 and 1985–86), along with one Scottish League Cup (1982) and two Scottish Cups (1980 and 1985). The latter Scottish Cup saw Provan write himself into football history, becoming only the third player at that time to score direct from a free kick in a Scottish Cup final. Celtic had been 1-0 down to Dundee United at the time, and following Provan’s equaliser went on to win 2-1.
Provan’s success at Celtic saw him capped for Scotland 10 times. He made his debut against Belgium in November 1979, scored against Israel in a 3-1 win at Hampden in a World Cup qualifier, and was part of Jock Stein’s 22-man squad for the 1982 World Cup in Spain although he did not actually play. Scotland were particularly well-off for tricky wingers in the early 80s, with the likes of John Robertson (Notts Forest), Davie Cooper (Rangers) and Peter Weir (Aberdeen) all providing Provan with stiff opposition for a place in the international side.
The start of season 1985-86 saw Provan in exceptional form for Celtic, resulting in intense media speculation that a recall to the Scotland international side was on the cards. However, Provan had to be substituted during a 3-0 defeat at Ibrox in November due to illness. This turned out to be M.E. – Myalgic Encephalomyelitis – a long-term condition that leaves the sufferer extremely lethargic and unable to perform much in the way of physical activity. Provan made several attempts at rehabilitation but eventually made the decision in the summer of 1987 to retire from playing football. His last game was coming on as a substitute against Motherwell in January 1986. A testimonial was arranged for Provan, and in November 1987 Celtic played Nottingham Forest in front of 42,000 fans, losing 3-1. Kenny Dalglish made a guest appearance for Celtic, with Provan playing for the first eight minutes.