 Johnston was promoted to Comptroller in 1981, replacing Lieutenant Colonel Sir Eric Penn shortly before the 1981 royal wedding of Charles, Prince of Wales and Lady Diana Spencer.As Comptroller, he took charge of protocol, state visits, investitures, garden parties, the State Opening of Parliament, royal weddings and royal funerals, administration of the royal palaces and the Royal Collection, the Central Chancery of Knighthood, the Lords-in-Waiting, the Gentlemen at Arms, the Yeomen of the Guard, the Royal Company of Archers, the Queen's Bargemaster, the Royal Watermen, and supervised swan upping.He also controlled royal styles and titles, matters of precedence, granting of Royal Warrants, and licensing the use of the royal arms.