Today, the Nottingham University graduate has a 20.7 million-strong following on YouTube, and is most popular among kids aged five to 10.
In the past year alone he has raked in a staggering A$25.4 million — or around $69,500 a day — thanks to his sellout live tour and merchandise including backpacks, baseball caps and hoodies.
Middleton has just been named as the fourth highest-earning YouTuber in the world, and the highest-paid in the UK, by Forbes magazine.
According to The Sun, Middleton previously said his astonishing success had been a shock.
"It's something I was just not prepared for at all," he said.
"It's something I've slowly been learning about in the past three or four years.
"In the past five years YouTube has evolved. Being a YouTuber was not a thing before."
His clips have been seen more than 10 billion times, with one video alone getting 44 million views.
And while an exact figure is unknown, Forbes estimates top online stars can earn around $6.90 for every 1000 views.
He has also been selected to voice the character eBoy in the film Ralph Breaks The Internet.
But as incredible as Middleton's success has been, it pales in significance compared to the top earner — a seven-year-old boy known only as Ryan, who posts videos of himself playing with toys under the channel Ryan ToysReview.
Ryan, from the US, made a stunning $A30.2 million in 2018, and has amassed 17 million followers plus has his own line of collectibles.
The boy, whose surname is unknown, has moved up from eighth position, which he held in 2017.
His family has been sharing clips of him playing with various toys since he was just three.
Meanwhile, second place went to Jake Paul, who attracts billions of views for his prank videos and rap songs.
Paul made $29.5 million this year, just ahead of "five-man sports crew" Dude Perfect, who took out third place with $27.5 million in earnings.
Fifth place went to makeup guru Jeffree Star, who has also launched a line of cosmetics.
Jeffree Star made $24.7 million from his YouTube channel and also rakes in a fortune from his Jeffree Star Cosmetics range.
According to Forbes, online gaming videos are now a bona fide trend, with WME digital-talent agent David Huntzinger telling the publication it was now "cool".
"Ten to 15 years ago, gaming wasn't cool. You didn't game because it was cool, you gamed because you loved it," he said.
"Now you have Drake going on Twitch and playing Fortnite, and athletes in the locker room saying they can't stop playing Xbox — it's what these kids are living and breathing."
Almost all the top earners in 2018 were aged in their 20s to 30s, with most also earning even more cash through their own merchandise lines.
The top 10 YouTube sensations earned a combined $248.3 million this year — a huge 42 per cent increase from 2017.
