Macari signed for Celtic at the age of 16. He quickly became part of the renowned reserve side known as the Quality Street Gang that also comprised the likes of Kenny Dalglish, Danny McGrain and Davie Hay. In August 1968, Celtic Reserves needed to defeat Partick Thistle Reserves by at least seven goals to win their Reserve League Cup section over Rangers Reserves. Celtic won 12-0, with Macari scoring four goals. Macari scored 91 goals in two seasons for the reserves and in occasional first team games, having broken through into the Celtic first team in 1970. In 1971 he replaced Willie Wallace in the starting line up for the replay of the 1971 Scottish Cup Final, and scored for Celtic in a 2–1 win over Rangers.
After a promising start to his playing career with Celtic, he moved south of the border in 1973 for £200,000 to sign for Manchester United, where he spent the bulk of his playing career. During his time with Celtic he had scored 57 goals in 100 appearances since making his first team debut in 1970. He won three League titles and two Scottish Cups in his time at Celtic.