The last round in Nairobi also proved memorable for his fellow Scot, Bob MacIntyre, as he signed off by breaking the course record at Karen Country Club with an eight-under-par 63.
That stunning effort lifted MacIntyre 40 spots as he finished in the top 20 for the third time in his rookie season after graduating from the Challenge Tour last season.
Johnston, who won twice on the second-tier circuit in 2018 to earn his step up to the main tour, was in the top 10 from start to finish in an event won by five Scots - Brian Barnes, Ken Brown, Garry Harvey, Craig Maltman and Mike Miller - when it was part of the Safari Tour.
The 26-year-old from Dumfries started the final round four shots off the lead and set out with high hopes of making his presence felt as he stuck to a gameplan of not having a driver in his bag.
A bogey at the par-3 second proved an early setback for Johnston and, though he recovered to make birdies at the fourth and sixth, it was all pars apart from one dropped shot thereafter.
A closing 71 for a for 10-under 274 total left him in eighth position, six shots behind Guido Migliozzi as the young Italian claimed his maiden victory on the circuit.
“Not the Sunday I was after,” admitted Johnston, “but eighth place is a PB for me on the European Tour so far. Now for a five-week break to recharge and prep for the rest of the season.”
The former Scottish Stroke-Play champion earned around £23,500 for his week’s work, which also lifted him to 102nd in the Race to Dubai.
MacIntyre sits 56th in the standings after his brilliant last-day performance, which came at the end of a nine-week stretch away from home for the 22-year-old from Oban.
“I’m absolutely delighted,” he said after a closing salvo that was illuminated by an eagle-3 at the 11th while he also six birdies in a bogey-free effort.
“I didn’t really expect that after the way I’ve been playing but I just went out with complete freedom and managed to get it round in eight under.
“It (his swing) just wasn’t feeling natural and today I went out there and made myself feel free and it just worked.
“I’ve got the game to go and shoot these numbers, as I showed today, it’s just playing with freedom and that’s what I did today. That’s the way I’ve got to try and get my mindset for the rest of the year.”
The former Scottish Amateur champion has made the cut in ten out of the 11 events he’s played in this season, including the last nine in a row.
Like Johnston, he’s now looking forward to spending some time back home before resuming his campaign in Morocco at the end of next month.
“It’s time to head home for about four weeks’ rest,” admitted MacIntyre. “I’m looking forward to the time off. I just can’t wait to get home and see the family and the two foster kids, Dan and Tom. It’s just so relaxing back in Oban.”
Migliozzi, a 22-year-old from Vincenza, closed with a 69 to win by a shot from Spain’s Adri Arnaus, who had held a share of the third-round lead, and South African pair Justin Harding and Louis De Jager.
Arnaus matched Migliozzi’s four birdies on the final day, but his three bogeys meant he had to settle for a 70 and a share of second place.
Migliozzi said on Sky Sports: “Today was a great battle. I like this moment, I like the pressure. It is like a drug. I like playing with a lot of people watching me and a lot of cameras.”
Playing in only his fourth European Tour event this season, Ewen Ferguson covered the last 12 holes in five-under as he closed with a 67 to finish joint-31st on four-under.
Following a third straight 70, Connor Syme ended up in a share of 35th on three-under, with Callum Hill closing with a 73 - his worst score of the week - to finish joint-66th on three-over.