The 2011 World Cup winner, 23, smoothly pulled away and never looked back to beat German Sabine Spitz, who came into the race as the defending champion, by 62 seconds.
American Georgia Gould took bronze, 68 seconds off the pace.
Bresset grabbed a French flag just before the line, waved it in celebration before lifting her bike over her head in delight, a broad smile on her face.
"Two days ago, I was practising my start and my front wheel slipped," Bresset said.
"I cut my arm and my knee, so two days before the start, I was not good at all. I needed seven stitches. I thought 'it's not possible'."
Bresset, however, regained her composure and went on the course again.
"I went back there to get a feeling of the course again," she explained. "I was still hurting but today that was OK."
France's first cycling title in London comes after three silver medals in the track competitions and dismal performances in the BMX.
It is also France's fourth gold medal in mountain bike since the event was introduced at the Games in Sydney in 2000. Julien Absalon will go for a third straight title in the men's race on Sunday.
Bresset, who had said she would need to take a flying start, teamed up with Britain's Annie Last to drag the field into the opening nasty climb in rural surroundings on a bright day in Essex, north east of London.
She quickly took control of the race, setting a hard pace in the lung-busting climb to Snake Hill before building a small gap over the pack along with Spitz and Canadian world number one Catharine Pendrel.
After the second of six 4.7-kilometre laps on a course overlooking the Thames estuary, the trio looked set to battle it out for the medals.
Pendrel, however, lost ground as Gould joined Bresset and Spitz at the front.
Nothing could deter Bresset from her goal.
"I was so focused all the time. I did not make a single mistake. I knew where I had to drink and where I had to eat," she said.
The Frenchwoman was still ahead after three laps and Spitz fell off her bike before recovering as she chased the leader in the tricky Rock Garden descent.
Asked if she surrendered her title with the fall, Spitz said: "Julie was quite strong, so it's just speculation.
"When I crashed I lost a bit of my rhythm ... but at that time Julie was already some 35 seconds ahead."
Bresset sat on a 33-second cushion with two laps remaining and it was then clear the only fight left was the one for silver, with Spitz retaining her small advantage over Gould on the line.
"I now have my complete collection of medals," said the 40-year-old Spitz, a bronze medallist in Athens.
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