Mullery became QPR manager in 1984 and guided the club to a 7–0 aggregate win over KR Reykjavik in the 1st round of the 1984–85 UEFA Cup, but he was then involved in an extraordinary 2nd round UEFA Cup tie against FK Partizan. In the first leg, which was played at Highbury because of UEFA's ban on the artificial plastic pitch at Loftus Road, QPR beat Partizan 6–2, despite being 2–1 behind at one stage and down to ten men after QPR defender Warren Neill was sent off. In the second leg, Partizan won 4–0 in Belgrade to win the tie on away goals. Partizan's victory is, to date, one of only three occasions in the history of European competition where a team has overturned a four-goal first-leg deficit.