In 2009, Johnston turned professional and won for the first time that year on the Jamega Tour. After a 2010 season split between the Jamega Tour, the
PGA EuroPro Tour and invites to the
Challenge Tour, Johnston enjoyed a breakthrough season in 2011. He played in his first
European Tour event at the
Alfred Dunhill Championship in
South Africa, and qualified for his first major championship, the
Open Championship, via local final qualifying at
Royal Cinque Ports. Johnston also recorded several good results from invites to the Challenge Tour, including a runner-up spot at the
M2M Russian Challenge Cup. That qualified him for the
Grand Final, where he finished in a tie for third to jump to 15th in the standings and secure a place on the European Tour for 2012. Johnston played 20 events in his first season on the European Tour, without being able to make any top-10 result and ultimately failing to retain his tour membership. In 2013, he was back on the Challenge Tour, where he registered a 5th place at the
Finnish Challenge and a 6th place at the
Northern Ireland Open Challenge, but failed to gain the European Tour membership by finishing only 68th in the final Order of Merit. At the end of the year, he entered the
European Tour Qualifying School; after winning the Second Qualifying Stage at Valle Romano Golf, he again came short finishing 52nd at the final event at the PGA Catalunya Resort. In 2014, Johnston finally found success in the Challenge Tour by winning two events, the
Scottish Hydro Challenge and the
Le Vaudreuil Golf Challenge; he ended up winning also the final Order of Merit and regained his European Tour membership. In his second season on the European Tour Johnston started with a 3rd place at the
Alfred Dunhill Championship, but struggled to make other positive results, missing 11 cuts in 25 events. At the end of the season he was able to retain his tour card thanks to a 10th place at the
Turkish Airlines Open. His 2016 started in January with a 4th place at the
Qatar Masters, after which Johnston earned his first European Tour win at the 2016
Real Club Valderrama Open de España. He went on to on secure two other top-10 results at the
BMW PGA Championship and at the 145th
Open Championship, where he enjoyed remarkable popularity for his look, personality and game. After the Open he entered for the first time in his career the top-100 of the
Official World Golf Ranking at the 89th place. In September, Johnston entered the
Web.com Tour Finals in an attempt to gain a
PGA Tour card for 2017; he achieved that at the second event of the series, the
Albertsons Boise Open, where he finished 4th. As a result, he got a card for the
2017 PGA Tour. In 2024,
HarperCollins published Johnston's autobiography,
Golf Is Hard. == Personal life ==