McCreadie started his footballing career with amateur Scottish side Drumchapel before moving to Clydebank Juniors and then East Stirlingshire. After turning down a move to Fulham, he was signed for Chelsea in 1962 by then-manager Tommy Docherty for £5000 to help the club's push for promotion from the Second Division. The club were promoted that year and McCreadie became a fixture in the Chelsea defence for the next decade.
A talented and pacy attacking full-back with impressive timing, McCreadie featured strongly in the flamboyant Chelsea sides of the 1960s and 1970s alongside the likes of Ron Harris, Bobby Tambling, Peter Osgood and Charlie Cooke. While he only scored five goals for the club throughout his career, McCreadie scored a memorable winner in the League Cup final of 1965 in which he dribbled 80 yards up the pitch before slotting the ball past Leicester City goalkeeper Gordon Banks to give his side a 3-2 first leg lead, which ultimately won the trophy for his club as the second leg at Filbert Street ended in a 0-0 draw.
After a string of high-league placings and near misses in the cups (including defeat in the 1967 FA Cup final) but no more silverware, McCreadie won the FA Cup with Chelsea in 1970, where a move involving him won the throw-in which created David Webb's winner in the replayed final against Leeds United at Old Trafford. The side won the Cup Winners' Cup the following season, but McCreadie missed the final in Athens through injury.
He was also a Scotland international, winning 23 caps between 1965 and 1969 after making his debut against England. He played in Scotland's famous 3-2 win over world champions England at Wembley in 1967, after which the Scots declared themselves the new unofficial world champions.
Upon his retirement from playing in 1973, McCreadie joined the coaching staff at Chelsea having made 410 appearances for the club. In April 1975 he was appointed manager but by this stage the team was in decline with the club heavily in debt and he couldn't prevent relegation to the Second Division. Nevertheless, McCreadie re-built the side - taking the captaincy from Harris and giving it to 18-year old Ray Wilkins in the process - and with no money to spend put together a team of youth players and veterans from the club's heyday. Chelsea were promoted back to the First Division in 1977. It was then that he lost his job in somewhat bizarre circumstances. Having won promotion, his request for a company car was rejected by chairman Brian Mears, so he resigned. Mears then relented and offered him the car but with his sense of Scottish pride he did not come back to the club as he had already made his mind up.
McCreadie left for the North American Soccer League in the late 1970s and was appointed manager of the Memphis Rogues, with whom he played one game in 1979, and later the indoor Cleveland Force before finally retiring from football in 1985. He continues to live in the United States.