By the age of 17, the bespectacled Australian was winning junior continental competitions.
Larkin was 19 at London 2012, making him the second youngest athlete on the Australian swimming team. He finished eighth in the 200m backstroke, but his journey towards bigger honours had only just begun.
Two years later, a physically more mature Larkin stormed to 200 metre backstroke gold at the 2014 Commonwealth Games in Glasgow, Scotland.
Silver medals in both the 50 and 100 metre backstroke followed at the same meet, forcing the swimming world to sit up and take notice of this rising star.
The Queenslander's big breakthrough came at the 2015 World Championships in Kazan, Russia.
Larkin followed that victory with another in the 200m backstroke, with 2014 Nanjing Youth Olympic Games sensation Evgeny Rylov taking bronze and London 2012 gold medallist Tyler Clary back in seventh.
A few months later, he broke the short course 200m backstroke world record.
Dubbed 'Larkinator' by his team-mates, he was the heavy favourite to claim Olympic titles at the Rio 2016 Games.
But the excitement was quickly extinguished as United States' breakout star Ryan Murphy promptly completed a 100m and 200m backstroke double.
Larkin was only fourth in the 100m after a slow start, but then edged out Rylov by one-hundredth of a second for 200m silver.
"I probably went into Rio knowing this was my best opportunity to come away with Olympic gold medal and I got silver." - Mitch Larkin to Swimming World Magazine
Larkin also took bronze in the 4x100m medley relay, while Murphy and Team USA claimed yet another gold.
Reflecting on 2016, the Aussie told Swimming World Magazine, "It was a fantastic year and I’m so grateful for it but I sort of said straight away it was probably the best thing and the worst thing at the time going into the Olympics as the favourite, all eyes are on you."
In a bid to rediscover his 2015 form, Larkin went through two coaches in the space of 12 months.
Then there was his break-up with Dolphins team-mate, five-time Olympic medallist Emily Seebohm, in 2018 which made a splash in the Australian tabloids for all the wrong reasons.
Eventually, the floundering swimmer found his way to Brisbane-based coach Dean Boxall, whose methods have since helped to resurface vintage Larkin.
“He’s like Superman,” Boxall told The Australian. “Superman needs a proper suit to fly because he is going so fast it can just break up on him.
“With Mitch there are just a few things we need to give him at training so he can have that proper suit. Once we give him that, he will just go.”
Under Boxall, Larkin was unstoppable at the 2018 Gold Coast Commonwealth Games, scooping a record five gold medals.
Among them was an unexpected win in the 200m individual medley, an event he had only recently taken up again after years of specialising in backstroke.
He broke his own 200 IM national record in qualifying at the 2019 Worlds in Gwangju, Korea, where 100m backstroke bronze and 4x100m mixed medley gold confirmed he was a force to be reckoned with once more.
"Finding myself again on an international level,” he told Digital Journal. "[There was] Almost a little bit of redemption."
With an extra year to improve and experiment due to the Tokyo 2020 Olympic postponement, the multi-talented swimmer is still undecided over whether to compete as a backstroker or to focus on his resurrected 200 IM career.
“I’ve done a little bit of medley work pre-season which is fun, it’s sort of back to my ground roots, and plenty of 4x400 IM sets which are nice and gruelling and that certainly builds the aerobic fitness and that was the plan,” Larkin told Swimming World Magazine.
“Dean and I we’ve always been of the opinion that one better swim is better than two average swims. So for me there’s no question of doing both so I’m definitely going to put all my eggs in one basket.”
With the 200m backstroke and 200 IM scheduled for the same day in Japan, he won't be following Ryan Lochte, who tried to do that double at Beijing 2008 and London 2012.
The American won the 200m backstroke in a new world record in Beijing before taking bronze behind Michael Phelps in the medley.
Four years later in London, Lochte was surprisingly beaten by Tyler Clary in the backstroke before improving to silver in the medley - again behind Phelps - half an hour later.
“It’s a very difficult double. Ryan is probably the only person who’s attempted it in recent years and I think unfortunately it cost himself an individual gold medal in those events." - Mitch Larkin to Swimming World Magazine
Larkin added, "I think I’ve got enough experience that I’m not worried at all. I’ve swum plenty of good medleys in my time and plenty of good backstroke and it’s just about tweaking that and I think the plan was to make mistakes last year and learn from them and make sure there’s no errors this year so we’re not guessing between the Trials and hopefully the Olympics."
Whichever path he chooses, he will not be under the same pressure he experienced ahead of Rio.
"This time around it’s completely different,” said Larkin of this third Olympic cycle.
"I’m a lot more freer and obviously know people are going to watch our trials quite closely and look at the result from there and whilst I’m not favourite certainly people will have an eye on me still."
Swimming is serious business in Australia and while Larkin is not a favourite going into the next Games, that doesn’t mean he is going for anything less than gold should he make the plane.
"It means everything for me. I grew up watching the 2000 Sydney Olympics and the likes of Grant Hackett and Ian Thorpe and the list goes on. There’s so many, you see them on the podium and as a young aspiring swimmer, that’s all you want and I guess that’s where the dream begins,” he said.
“If you had told me in 2016 you would retire without an Olympic gold I would have been pretty upset but now that thought doesn’t upset me as much knowing I’ve given everything I can in the sport and I’ve taken a lot more away than just individual medals, which I’m pretty proud of.
While his event is still in doubt, there is little to be questioned over Larkin's improved head space and happiness compared to the darkest depths of 2017.
With a wealth of Olympic experience to now call upon, perhaps it will be third-time lucky for Australia's Renaissance Man.