The Englishman came into the final day alongside Luiten at two over and signed for a one under par 70 to take the title by one shot and become the first man to win a European Tour event with an over par score since Justin Rose won the US Open at Merion in 2013.
It had been 20 years since a regular event had been won over par, with Ian Woosnam shooting the same score to win the Scottish Open at Carnoustie in 1996, showing just how difficult a test Valderrama had been over the four days.
Rain had softened the greens, with lighter winds making birdies more possible in the final round, and Johnston made what proved to be the vital gain on the 16th.
“I drove the ball well all week, chipped and putted well all week, I struggled a bit with my irons the first three days but I just kept going and kept digging in and then on the fourth I found something,” he said.
“I came off the course and I just started crying with the emotion of it. It’s just very surreal and I don’t think it will hit me for a few days.
“You see them winning these tournaments, and the history of it and all these names, so to put mine on there feels pretty ridiculous, really. It really does.
“Hopefully I can be one of those guys who has an incredible career like that and people are sitting here in the future going, ‘oh, Beef’s on there’. It’s pretty incredible.”
Tournament host Sergio Garcia made six birdies in his round of 67 to finish at three over, a shot ahead of defending champion James Morrison and Søren Kjeldsen.
The win completes a long journey for 27 year old Johnston, who first made it onto The European Tour via The Challenge Tour in 2012 before a loss of form and fitness saw him lose his card.
Two wins on The Challenge Tour in 2014 saw him win The Road to Oman and now, after finishing 70th on The Race to Dubai last season, he has his breakthrough.
For Dutchman Luiten, the result is the continuation of a brilliant start to the season and hands him his fifth top ten in eight events so far this term.
Johnston bogeyed the first as those around him jostled for position, with Martin Kaymer opening up a two-shot lead over the field with a birdie on the third.
Luiten and Kaymer continued to battle it out but when Johnston hit a sensational approach to the seventh for birdie and made another gain on the next, a dropped shot from the German on the seventh meant Johnston and Luiten shared the lead at the turn.
Morrison had also turned in 34 and bogeys on the 11th from Luiten and 14th from Johnston meant there was a three-way tie for the lead. That became a four-way tie when Garcia birdied the 16th and 17th to surge into contention.
A bogey on the last saw Garcia set the clubhouse target at three over but Johnston holed a 20-footer on the 16th to move ahead and Luiten could not reel him in as he finished with seven straight pars.
Kjeldsen’s 68 handed the Dane a second straight top ten after his excellent performance at the Masters Tournament last week, while Morrison finished with a 72.
Ross Fisher, Kaymer, Mike Lorenzo-Vera and Alex Noren were then at five over, a shot clear of Craig Lee.