Uchimura ended on 91.500 points, a massive 2.575pts clear of Great Britain's Daniel Keatings, who thrilled the home crowd by claiming the silver medal.
Russia's Yury Ryazanov, a further 0.525pts back, picked up the bronze, denying Japan's Kazuhito Tanaka a finish on the podium alongside compatriot Uchimura.
Uchimura, who as a teenager became champion of Japan in the all-around competition last November, never looked likely to be stopped after grabbing the lead following the second discipline, the pommel horse, an apparatus he struggled on in Beijing.
He was so far ahead by the fifth and sixth rotations that his procession to gold was all too predictable. By then, all eyes had turned to the battle for the other medal positions.
"Today's performance wasn't entirely up to my best standard," said Uchimura. "I'm not completely satisfied but I got the result."
"I didn't think I would have a chance of winning a medal at the Olympics. But after winning silver, I was aware of my position in the world standings and I continued to work hard and thought that the gold medal would be reachable here.
"I hope to return in three years' time for the Olympics here. It's been a great experience and I hope to come back and repeat it."
Keatings ended up comfortably holding on for second place, becoming in the process the first British male to win a medal in the all-around event at a world championships.
It is the first clear sign of Britain's emergence in world gymnastics ahead of the London Olympics in 2012.
Ryazanov sneaked bronze on the final discipline, leapfrogging the unfortunate Tanaka.
With China's Yang, the 2007 world champion in the all-around competition, having retired, there was always going to be a new winner of the title.
Fabian Hambuchen, silver medallist in Stuttgart two years ago, had been tipped to do well but he injured his left ankle in training on Sunday and is currently on crutches.
So Uchimura came into the finals as heavy favourite and started confidently, excelling as usual in his opening floor routine and then showing great form throughout on the pommel horse, which he fell from in Beijing.
He never relinquished a lead he earned after his exhibition on the pommel, and following a problem-free showing on the rings in the third discipline he was almost 1.5pts ahead of nearest rival, Colombia's Giraldo Lopez.
Uchimura nailed his landing on the vault - the fourth apparatus - to further dent his rivals' hopes and confidently made his way through a faultless performance on the horinzental bars to secure gold.
The Japan star was the best performer on the floor, the vault and the rings at the O2 Arena, which will host the artistic gymnastics at London 2012.
Keatings, 19, gave the home crowd, which filled about three quarters of the lower tier at the venue, something to shout about when he crept into second place overall by the fourth rotation.
And he put in a sublime performance on the horizontal bars - one of his favourite disciplines - to bring the house down and seal a deserved second place.
"Uchimura is the best gymnast in the world but I'd like to think I can get to his standard," said Keatings.
Russia's Maxim Devyatovskiy came fifth, 0.825pts behind Tanaka, and Kristian Thomas, of Great Britain, was sixth.
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