It also continued a fine run of form since last season’s Open Championship, with ten top 20 finishes in his last 14 starts and 42 rounds of par or better in his last 43.
Johnson finished with an eagle on the last in a closing 68 with Larrazábal signing for the same score after a roller coaster of a final round.
Austria’s Bernd Wiesberger also signed for a 68 in finishing at 15 under alongside three-time champion Martin Kaymer and Thailand’s Kiradech Aphibarnrat, who had rounds of 69.
Fleetwood had one top ten in 14 months following the 2015 Aberdeen Asset Management Scottish Open and is delighted to at last see the hard work pay off with the help of good friend Ian Finnis on the bag.
“I thought maybe the second win would come sooner to be honest,” he said. “It’s been a massive comeback.
“I made it up to Finn, who has been caddying for me, one of my best mates. We made the switch and not a lot of people thought it would work and we proved them all wrong. It feels great.
“I had a really hard time from July 2015 to July last year where I really struggled with my game. It’s been an awkward curve.
“The only thing left was ‘let’s get a win’ but, you know, you keep knocking on the door and it will come. I didn’t really think it would be this week. It just shows a lot can happen in golf.
“I just thought, keep going, you never know. It’s a tough day really. The pins are tucked away and it’s very windy, especially at the start. The chip-in on ten, birdie on 11, changed everything all of a sudden from nowhere and I was leading.”
Fleetwood bogeyed the third and birdied the eighth on his front nine but it was playing partners Kaymer and Aphibarnrat who were battling for the lead.
Aphibarnrat made a birdie-birdie start in turning in 33 and with Kaymer also playing his front nine in the same score with the help of three birdies from the sixth, they led at the turn.
Trouble off the tee and a poor approach saw Kaymer fall from the summit on the tenth and Fleetwood was in position to take advantage, holing out beautifully from just in front of the green.
Another birdie on the 11th saw him join the lead and Aphibarnrat did well to keep the damage to a single shot after finding the water on the 12th before bouncing back with a birdie on the next.
Larrazábal had five birdies and three bogeys in his first 13 holes before a gain via a sublime chip-in on the 14th moved him into a share of the lead.
A wonderful approach from Fleetwood on the 16th then sent him to 16 under and a birdie on the last left Larrazábal needing an eagle in the group behind to force a play-off.
The Spaniard got free drops off his first and second shots but could not hole his pitch and Fleetwood had another trophy to add to the 2013 Johnnie Walker Championship at Gleneagles.
Johnson began his round with seven pars and while he picked up shots on the eighth and 13th, a bogey after finding sand on the next effectively ended his chances.
Kaymer joined Johnson with an eagle on the last for his seventh top ten in 11 events here, while Wiesberger had birdies on the eighth, tenth, 11th and last in a bogey-free effort.
South African Dean Burmester closed with a 69 to finish 14 under, a shot clear Peter Hanson, Henrik Stenson and Lee Westwood.