The 39-year-old, who works as an electrical design engineer, secured the biggest win of his career with a two-hole victory over Dunbar’s Lee Morgan in the final at Duddingston.
Johnstone, who plays off plus one, was two up after 10 holes before losing three holes in a row to the 2016 Lothians champion.
A good birdie chance for Johnstone then went abegging at the 15th, but he made amends with back-to-back birdies at the next two holes to get his nose in front again.
A wedge to four feet did the trick at the par-4 16th before he was safely on in two at the long 17th, winning the last as well with a par to seal the deal.
“It’s not sunk in yet, especially as I keep looking at the names on the trophy,” said Johnstone, referring to past winners having included the likes of Ronnie Shade and George Macgregor.
“I played in this event a number of times and to win it in the centenary year of the Lothians Golf Association is very special.”
Johnstone had beaten Steven Sinclair 2&1 in the semi-finals earlier in the week while Morgan overcame the host club’s Ben Alexander by 3&2.
“I only had one home game and, in the second round, I beat Benn McLeod at Monktonhall, so it was a good effort,” added the new champion.
Pumpherston’s Billy Bishop won the handicap event, beating Peter Whitecross of Liberton in their title decider at the same venue.
“This was only my second appearance in the event, having been knocked out in the first round a few years back,” said Bishop.
The 51-year-old, who lives in Fauldhouse, had his first hole-in-one en route to the final when winning an away tie at Gullane.
Meanwhile, Broomieknowe’s Hannah Darling will find out on Monday if she’s done enough to secure a Curtis Cup appearance at Conwy in Wales next month.
The Lothians star is among the contenders for the GB&I team, having been in great form since winning the St Rule Trophy in St Andrews earlier in the year.