A clear round in the tense show jumping finale was enough for the Hallington rider and her mount Allstar B to take the individual title and anchor the Great Britain to team gold.
Tension mounted in Tryon, North Carolina, with Canter heading into the show jumping arena placed second after dressage and cross country.
Going out as the penultimate rider she needed a clear round to maintain her position, and no more than one fence down to clinch team gold for Britain for the sixth time.
The Lincolnshire combination showed nerves of steel to go clear under intense pressure and seal, at least, the silver medal.
It seemed the 32-year-old would have to settle for second-best as overnight leader Ingrid Klimke looked destined for a clear round.
But there was agony for the German when her horse SAP Hale Bob OLD clipped a rail on the final fence to promote Canter into gold in the most dramatic of finales.
“I don’t think it’s sunk in, I can’t believe it,” said Canter, who became only the fifth Briton to become world eventing champion.
“Allstar B was absolutely amazing, he was an absolute hero, I had an amazing experience in there.
“I kept saying [to myself] just let him do his job, and I’m so proud, he’s just phenomenal.
“There were quite a few tears when I found out which isn’t normal for me.”
Allstar B took little time in adapting to the heat on his first trip to the USA, and pulled off a mistake-free dressage test to sit third on 24.6.
On Saturday’s cross country all of Britain’s riders went clear around the cross country course to elevate the team into gold medal position and Canter into second place.
They were given an extra day to withstand any nerves when Sunday’s show jumping was postponed for 24 hours as Hurricane Florence approached.
By the time Canter entered the arena, she had a fence in hand to secure the team gold, but only a clear would keep her ahead of Irish rider Padraig McCarthy.
But they kept their cool to become only the fourth British combination ever to win world eventing team and individual gold - and the first since 1986 to do the double.
It has been a sensational rise for the horse and rider who made their four-star debut just three years ago.
A fifth-place finish at Badminton earned an international debut at last year’s European Championships where they were the highest-placed British finishers in fifth.
And in just 13 months the pair have sprung to the top of the world, with the Tokyo Olympics in two years a tantalising prospect.
“It’s a surreal day for me,” she added. “I’m proud for my country and excited for the whole team around me and everyone who’s put so much into this.
“It’s just the most incredible feeling.
“The team around us is just phenomenal. They make the dream come true really.”