Scottish born Johnston began his career as a vet, studying at Glasgow and practicing for three years before buying a training yard in Louth, Lincolnshire in 1986. He obtained his trainer’s license a year later, achieving his first win with HInari Video at Carlisle.
In 1988 he moved to the Kingsley House yard in Middleham from where he built one of the most successful stables in the country. Eventually extending it to two further stables, the training facility now covers 270 acres.
Jonhston’s first British Classic win came in the 2,000 Guineas with Mister Baileys in 1994. In 2004 he won the British and Irish 1,000 Guineas with Attraction, who he cites as the horse he’s proudest of having trained. His most successful was Shamardal, who was voted the 2004 European Champion Two-Year-Old. During his three seasons, Shamardal won the Vintage Stakes at Goodwood, Dewhurst Stakes at Newmarket, The Poule d'Essai des Poulains at Longchamp, The Prix du Jockey Club at Chantilly and the St. James's Palace Stakes at Ascot.
In 2017 Jonhston became only the third trainer to pass the 4,000 British winners milestone. He currently sits behind Richard Hannon Snr and Martin Pipe on the all-time list of British winning trainers, who have 4,193 and 4,183 wins respectively.
He cites Mister Baileys win at the 2,000 Guineas as his career highlight, claiming the victory enabled his career to move up “another level”.