Macari played for Manchester United in the 1970s and early 1980s. He was bought from Celtic in 1973 for £200,000 during his time with Celtic he scored 57 goals in 100 appearances. His first game for the club came in January 1973 against West Ham United in which he scored a point-saving goal in a 2–2 draw. In 1977, his shot deflected off team mate Jimmy Greenhoff to win the FA Cup final against Liverpool (and ultimately deny Liverpool the European treble). He made 400 appearances for the club and scored 98 goals. Macari's early career at Old Trafford was spent trying to lead an attack that struggled to achieve anything. Relegation to the Second division in 1974 was the low point but Macari blossomed as a midfielder in the following seasons under Tommy Docherty as United began to win back a large following with superb attacking football in which Macari enjoyed popularity alongside players such as Gordon Hill, Steve Coppell and the Greenhoff brothers. He helped United win the Second Division title in 1975. They finished third on their return to the top flight and were runners-up in the FA Cup before going one better and lifting the trophy a year later. He was on the losing side in the 1979 final, and also played in a string of European campaigns during the late 1970s and early 1980s.