Harrison, 22, the world number two who had been aggressive and dominant throughout the competition, took the title beating Britain's rank outsider Gemma Gibbons in the women's -78kg category in a close final with two scoring throws.
"To be honest I was nervous to be fighting a British person with that crowd out there. I knew that today was my day. I knew that if I was going to lose she was going to have to take it from me," Harrison told reporters.
She said the gold medal represented years of hard work, not least to get over the childhood trauma of suffering sexual abuse. She thanked her "hero", her coach Jimmy Pedro, who won two bronze medals for the United States in Athens and Atlanta.
"It's no secret that I was sexually abused by my former coach and that was definitely the hardest thing I've ever had to overcome," she said. "We all sacrificed a lot for this and it means the world to me."
Prime Minister David Cameron, who was in the audience with Russian President Vladimir Putin, commiserated with Gibbons, Britain's first judo medal winner since Sydney in 2000, as she left the mat.
"I didn't know they were there," Gibbons said. "He just shook my hand and said 'fantastic, well done'.
"At the moment, the whole day its a bit of daze but I think I'll look back in weeks, months and years to come and think what a great moment that was."
Gibbons had surprised everyone making it to the final, having never won a medal at a major tournament before.
She overcame France's Audrey Tcheumeo, the world champion and ranked third in the world, in a titanic semi-final battle in which neither fighter gave an inch, hurling her more heralded rival to the mat for a winning ippon in extra time.
"I love you mum," she mouthed as she sank to her knees and looked to the heavens. Her mother, who introduced her to the sport, died from cancer in 2004.
"I want to say thank you and obviously I can't," she said. "There has been a lot of hard work and a lot of ups and downs, even in the last few years, and it's all come together at the best possible time."
Her boyfriend Euan Burton, who is another of Britain's judo team but lost his opening match two days ago, was among the noisy, cheering crowd at the London ExCel Centre, just a few miles from Gibbons's home in Greenwich.
"I caught his eye during the contest in the final," she said. "I still think he's much better than me," she laughed.
Tcheumeo, 22, recovered from her shock defeat to win France's sixth judo medal of the Games by defeating Abigel Joo of Hungary for bronze.
Brazil's Mayra Aguiar, who celebrates her 21st birthday on Friday, was delighted with an early present of the bronze medal she secured by beating Marhinde Verkerk of the Netherlands. It was the Brazilian team's third judo medal.