And so the New Zealand Children's Commissioner at the time, Dr Cindy Kiro, wrote Mitchell a speech on behalf of all New Zealand children.
Mitchell remembered being "quite nervous" before it was his turn to take the stage, but once he got up there all his worries melted away.
On the spur of the moment, Mitchell decided to give Ali his own greenstone pendant and tell him about what it meant.
"I just thought it was a good gift to give because it's from New Zealand and it's a special gift that represents strength and courage," he said.
Ali then gave the young Kiwi boy a hug as a thank you.
"It was pretty special."
Mitchell also presented Ali's wife, Lonnie, with a framed bone carving of a koru from the Children's Commission on behalf of New Zealand children.
When he got home, Mitchell was a star - he was the boy who had met Ali.
To this day, when people enter the Butlers' Greenlane home, one of the first things they see is framed copy of the Herald's front page showing a grinning Mitchell hugging the boxer.
"It was really a once-in-a-lifetime thing."
In December, Mitchell learned that he had been invited to film a video birthday greeting for Ali's 70th birthday party tomorrow (NZT) - with input from other well-known New Zealanders.
Prime Minister John Key and golfer Michael Campbell appeared on the video and wished the former heavyweight champion a happy birthday.
Mitchell, about to enter Year 11 at Sacred Heart College, said it was "pretty exciting" being invited to wish Ali a happy 70th birthday.
"I was pretty happy that Ali had remembered me and that I'd left a bit of an impression on him. So it was good that he thought of me so I could say happy birthday. It was good."
Mitchell, who doesn't box but does play cricket and rugby, still considers Ali one of his personal idols for what he achieved and the manner in which he achieved it.
"Muhammad was obviously an iconic sportsman that had a great personality and left a big impression on my dad and on me now."
He even has the middle name, Clay - after Ali's original name, Cassius Clay - to prove his and his father's admiration for the boxer.
Mitchell was born a few months after a shaking Ali ignited the Olympic flame at the 1996 Atlanta Games, weakened from his long battle with Parkinson's disease. The moment inspired Mr Butler to name his son in part after the boxer.
Mr Butler has followed Ali since watching his 1974 upset win over George Foreman to reclaim the heavyweight crown in the "Rumble in the Jungle".
"We were sitting there watching this fight. And there was this guy called Muhammad Ali who was absolutely getting beaten up by George Foreman, but it was all part of his tactics and he came through and won.
"It was one of those moments that you remember quite vividly."
Mitchell's birthday video will be shown at a private birthday party tomorrow evening at the Muhammad Ali Centre in downtown Louisville. Ali's actual birthday is on Tuesday.
The party will double as a fundraiser for the centre.
