Riner fended off Russia's European champion Alexander Mikhaylin to claim gold as the Tricolors waved around a packed 7,000-strong arena.
However, the nation which gave the martial art to the world headed home to dissect their worst performance since Judo first appeared at the Games in 1964 - and also the only time they have failed to deliver a gold for the men.
It was a resurgent Russia who topped the final Judo medal table, with three gold, one silver and one bronze medal, all of them from the men's events - which delighted Premier Vladimir Putin, a Judo black belt, when he visited the event alongside British Prime Minister David Cameron.
Although Japan won the most medals alongside France, they collected just one gold, from Kaori Matsumoto in the -57kg after both lightweight favourites Tomoko Fukumi and Misato Nakamura had failed to produce the goods, as did Yoshie Ueno in the -63kg, where Urska Zolnir won Slovenia's first ever Judo gold.
Men's -73kg fighter Riki Nakaya, world gold medallist in 2011, had to settle for silver, while Masashi Ebinuma, world champion at -66kg, could only take home bronze.
It was not a good competition for returning champions as none of the seven Judoka defending titles from Beijing stood top of the podium again in London.
Rankings also offered little safety on the tatami as the seeds tumbled, with fancied Italian Giulia Quintavalle and Yang Xiuli of China failing to claim medals, while Azerbaijan's Elnur Mammadli, who had won Olympic gold four year ago in -73kg, crashed out of the -81kg in the first round.
Outsiders ruled as 20-year-old Lasha Shavdatuashvili of Georgia upset the top contenders to take -66kg gold, while veteran Song Dae-nam claimed the -90kg crown and Antoine Valois-Fortier earned -81kg bronze - the first medal for a Canadian Judoka in 12 years.
The 2012 Judo tournament was also a landmark for the Host Nation, with a silver for Gemma Gibbons from the -78kg ending a 12-year wait for Olympic success, which was swiftly followed by a bronze for veteran heavyweight Karina Bryant.
The final word, though, went to Judo's gentle giant after Riner, 23, made up for bronze in Beijing with another golden display.
'For the last four years I have been thinking of this medal,' he said. 'I think I will sleep with it tonight.'
For the last four years I have been thinking of this medal. I think I will sleep with it tonight.Teddy Riner