Johnston began the day one shot off the pace, but after a 1-under 70 he ended up with a one-shot win over Joost Luiten. Despite overnight rains softening conditions, Valderrama Golf Club still provided a stern test and Johnston's 1-over 285 total is the first over-par winning score on the European Tour since the 2013 U.S. Open at Merion.
The last non-major on the European Tour with an over-par winning score was the 1996 Scottish Open, won by Ian Woosnam at Carnoustie.
''I can't wait to get back to North Mid (North Middlesex Golf Club), get hammered and see my mum and brother and see them and just celebrate,'' Johnston told reporters after his round.
Johnston was steady during the final round, but he received a number of challenges - notably from tournament host Sergio Garcia. The Spaniard charged up the leaderboard during the final round, shooting a 4-under 67 that served as the low round of the day. But Garcia bogeyed the last, and at 3 over he finished two shots behind Johnston.
Two-time major champ Martin Kaymer was also in the mix late, with a chance to tie for the lead with an eagle on the par-5 17th. Instead, his 7-iron approach found the water short of the green, leading to a bogey that ended his title hopes.
Even more surprising was the demise of Pablo Larrazabal, who was two shots back through 69 holes. But Larrazabal made triple bogey on No. 16 followed by a quintuple-bogey 10 on the 17th, and after a closing bogey he signed for a final-round 79.
Johnston's closest challenge came from Luiten, who needed to birdie one of the final two holes to force a playoff. Instead he closed with a pair of pars for his fifth top-10 finish in eight starts this season.
It left the trophy to Johnston, 27, who now has his first win in his 63rd European Tour start. He entered the week at No. 233 in the world with only one top-10 finish this season, and he hadn't played competitively since a T-15 finish at the Perth International in February.