Cancan, the world champion and number one seed going into the fight, started as a favourite but 29-year-old Adams, seeded second, had impressed all week and had home advantage in her favour.
Cancan had beaten Adams twice at world championship level, but Adams had shaded her 12-11 in their most recent bout, and started brightly in the first of four two-minute rounds, taking it 4-2.
The deficit inspired Cancan to come out firing at the start of the second. Adams, happy to trade, looked equally comfortable with the change of pace, with a right-cross catching her opponent flush, before knocking her down moments later with a right, and stretching her advantage to five points.
The third round was more sedate, with Adams looking assured and countering her opponent's every shot with blows of her own.
With a nine-point lead going into the final round, Cancan needed something extraordinary to turn the bout on its head, but Adams maintained her discipline to secure the victory by 16 points to seven, delighting the ExCeL Arena, and giving Britain their first boxing gold medallist of the Games.
The Briton shadow-boxed in each corner of the ring in delight after the official decision was declared.
Losing semi-finalists Mary Kom of India and American Marlen Esparza took bronze.
“I’ve just wanted this all my life. To think I’ve finally done it and I’m finally here, with all this support, it’s really made my day,” said Adams.
“The second round knockdown set me on my way and I really didn’t see it coming. It’s just one of those things, it’s amazing.
“I’d just like to thank all the supporters here and all those back home and I’ve brought that gold medal back to Leeds.
“The feeling of the gold medal is absolutely amazing and to see that flag being raised, I can’t wait for it to happen – I’m so overwhelmed right now.”
British boxers have already guaranteed themselves another four bronze medals as Freddie Evans, Anthony Ogogo, Anthony Joshua and Luke Campbell secured themselves a fight for the gold medal.
Ren was not downbeat following the loss, embracing the historic moment for the sport.
"I am very happy," she told reporters. "It was a very special moment."