Mickelson, less than a month shy of his 51st birthday, captured the PGA Championship to smash the record as the oldest major winner previously held by Julius Boros, who won the 1968 PGA at 48.
Getson, a former Australasian and Asian Tour player, has worked with the now six-time major champion since 2014 having met him in Scottsdale, Arizona while part of the Grayhawk Golf Club.
He's been a significant presence by the veteran's side on the US PGA Tour since.
"His guidance has been invaluable, really, to get me back to playing at the highest level because I was striking it very poorly when we started working together years ago, and I had a great performance (after that) at the 2016 Open Championship where I lost to Henrik Stenson," Mickelson said after his famous win.
"When he's out here with me, he's able to keep me on track right away if I make a few errors.
"He's really helped me get my ball-striking back and as I'm starting to focus a little bit better, you're starting to see the results, but he has been getting my swing there for some time now.
"He is a tremendous instructor because of his ability to simplify it. He has helped get my swing on plane from parallel to the ground.
"Obviously I have a long swing but rather than try to change that when it's halfway down … he helped me develop and refine my feel and touch and simplify it. He doesn't cloud my head with a lot of things."
Getson notoriously shies away from media when it comes to his work. He prefers his clients get the attention.
"It's a special and emotional day," he reluctantly told AAP in the aftermath.
"He's worked really hard for this and deserves every accolade he gets."
Getson joins Cameron McCormick and Colin Swatton as Australian coaches to guide players to major championship success in the past six years.
McCormick helped Jordan Spieth to three majors (2015 Masters, 2015 US Open, 2017 British Open) while Swatton was at the helm when Jason Day won the 2015 PGA.