Johnstone had a morbid fear of flying, which Jock Stein used to great effect on one occasion. Ahead of the first leg of a European tie against Red Star Belgrade at Parkhead in November 1968, Stein told Johnstone that, if Celtic won by four goals, he wouldn't have to travel to Yugoslavia for the second leg. Johnstone went on to produce an outstanding performance, scoring twice and providing assists for the other three goals in a 5–1 win. He helped Celtic reach a further European Cup Final in season 1969–70. His performance in the second leg of the semi-final against Leeds United at Hampden Park was particularly outstanding, and his mazy run in the second half set up Bobby Murdoch to score with a powerful shot for Celtic's winning goal in a 2–1 victory on the night. Leeds' defender Terry Cooper later quipped "I would love to have kicked Jinky, but I couldn't get near him!" Billy Bremner described Johnstone's performance as "one of the greatest exhibitions I have ever seen", adding that "Jimmy had one of these games where he was unstoppable." Celtic played Feyenoord in the final but, with Johnstone double-marked at all times, Celtic were unable to perform as they had in the 1967 final. Feyenoord dominated the match and defeated Celtic 2–1 after extra-time.