The 25-year-old Dumfries man stood four shots behind both Germany’s Jonas Kolbing and Antoine Schwartz of Italy when he arrived on the 16th tee in the final round of the 54-hole event in Casablanca.
But Johnston’s 3-3-3 sign off on the Tony Jacklin-designed Prestigia Golf layout, coupled with Kolbing and Schwartz both dropping two shots over that closing stretch, saw the Scot come out on top in a dramatic finish.
The late burst gave Johnston a 12-under-par 203 total, having stormed into contention after picking up eight birdies as he raced to the turn in 27 en route to a second-round 64.
“I’m delighted to get the first win on this Tour so early in the season,” admitted Johnston. “Being four back with three to go, I was just trying to finish as high as I could.
“But to hole a 12 foot putt on the last to win was very pleasing. It was hard to stay patient early in the round being four or five back all the time but I just tried to hang in there and in golf anything can happen!”
Johnston won both the African Amateur Championship and Scottish Open Stroke-Play Championship last year before turning professional.
In his first outing in the paid ranks, he birdied the last at Carnoustie to make the cut in the Dunhill Links Championship before closing with a 68 at St Andrews to finish joint 47th, earning around £15,500.
He then won the ProGolf Tour Qualifying School in Germany and had already got his 2018 campaign off to a promising start by finishing runner-up in the season’s opening event, the Red Sea Egyptian Classic, last month.
Johnston, who graduated with a psychology degree from the University of Tennessee at Chattanooga, is coached in Dumfries by James Erkine.
He joins Paul O’Hara and David Law in becoming a winner on the ProGolf Tour, a third-tier circuit that is associated with the PGA of Germany.
Elsewhere, an English victory looks on the cards in the £35,000 Palm Hills Championship, a Mediterranean Golf Tour event in Egypt, after Liam Harper added a 65 to his opening 69 to sit five shots clear on 10-under.
Ross Kellett (72-76) is the leading Scot heading into the final round, followed by Michael Stewart (75-76) and Michael Stokes (80-81).
In the amateur ranks, Cawder’s Calum Fyfe fared best among five Scots in the field in the opening round of the South African Stroke-Play Championship at Pecanwood Golf & Country Club in Hartbeespoort.
In the first event of the Scottish Golf squad’s annual winter training trip to the Rainbow Nation, Fyfe carded a five-under-par 67 to sit three off the lead, held by home player Keaton Slatter.
Sam Locke, the Scottish champion from Stonehaven, and the ever-improving Darren Howie of Peebles backed up their compatriot with 69s while Euan Walker (Barassie) also broke par with a 70.
John Paterson, last year’s Scottish Boys’ Stroke-Play champion from St Andrews, had a day to forget as he slumped to a six-over 78 to sit joint-last in a 140-strong field.