His career included some controversial incidents. Widely regarded as possessing a short temper to match his quickness and skill, he was sent off over 20 times in his career. His move to West Brom came in the wake of a two-month ban for punching an opponent. After Scotland's opening game against Peru in the 1978 World Cup in Argentina, Johnston tested positive for a banned stimulant contained in Reactivan, a medication prescribed for his hay-fever. His subsequent expulsion from the squad ended his international career after 22 appearances, although he maintained his innocence. A year earlier, having been provoked by his opponent, he had been shown a red card for violent conduct in a friendly against Argentina on his last visit to that nation.