It was a world-record setting performance – by a winning margin of 1.56 seconds no less – that will ensure his life will never be the same again.
But while his pre-race calmness, untouchable speed in the pool and composure on the medal rostrum afterwards seemingly belied the fact that the 21-year-old was competing at his very first Games – it was all part of the master plan for one woman deep in the Rio Olympic Aquatics Centre.
"I absolutely knew he was capable of that but we just had to keep it a secret," said Mel Marshall, a coach at the City of Derby and most significantly, mentor to Peaty.
"I know what it's like on the other side if it doesn't go well so there was an element of being cautious but in my gut, I knew what he could do. I fully trusted in him and I fully believed in him, He was supposed to win this, it was destiny."
That unwavering faith in her athlete stems from a relationship the two have enjoyed since Marshall first spotted 'a sparkle' in the then 14-year-old Peaty. Marshall had already enjoyed her own successful swimming career and the influence of the 34-year-old on Peaty's career is immeasurable.
At Athens 2004, Marshall had headed into the Games ranked world number one in the 200m freestyle event but then failed to make the final – although she admits that experience has helped mould not only her as a coach but Peaty's elevation to the top.
Adam Peaty of Great Britain poses with his gold medal
Image credit: Reuters
"I feel like there is a massive amount of fate gone on with this journey," she added.
"There were so many situations – I had just come out of an elite sport from Beijing and retired – and he had just landed on my doorstep and it went from there. Eight years ago I dreamt of this and now we're here.
"If I look back now at Athens 12 years ago I would say that was my defining moment as a coach, I would never have been able to guide him as a coach if I had not had that experience myself. I feel like this for me was my Olympic gold."
Given the amount of work she has dedicated to helping Peaty reach his full potential at such a young age, it would be churlish not to let Marshall also enjoy her moment in the spotlight. However the role of any good coach is to step back when the moment demands, and Marshall was quick to lavish praise on her athlete, who is now holds the Olympic, World, European and Commonwealth 100m breaststroke titles.
"My final words to him were use you speed with a great stroke, use your courage with a great stroke and go and make some history," said Marshall, who added that the next target would be breaking the 57 second barrier.
"He's always the last man standing no matter what it is. It's a testament to his hard work that he's here. He will always listen and that's one of the good things about him. I feel like we're the perfect partnership and that's a special and a rare thing. I will never forget this moment for the rest of my life."
Neither will most of Great Britain.