Ennis, who has been the face of the Games, finished like a champion by winning the 800 metres for a combined total of 6,955 points for the seven disciplines that make up the event.
Only American world record holder Jackie Joyner-Kersee, Sweden's Carolina Kluft and Russian Larisa Turchinskaya have scored more.
There was confusion over the minor medals with Russian world champion Tatyana Chernova initially being awarded the silver and Ukraine's Lyudmyla Yosypenko the bronze after German Lilli Schwarzkopf was disqualified from the 800 for a lane infringement.
However, Schwarzkopf won her appeal and was reinstated in the race which gave her the silver and moved Chernova down to bronze.
"I honestly can't believe it, after all the hard work and after the disappointment in Beijing. I am just so happy," said Ennis who missed the 2008 Olympics with an ankle injury.
"I want to thank everyone who has supported me, they have been amazing," she told reporters.
"I just had to give it everything at the end. I just wanted to make sure I gave them something and bought it all home."
Ennis led the field following Friday's four events and all but secured the gold after finishing second to Chernova in the long jump and setting a personal best of 47.49 in her weakest event, the javelin.
"I was really nervous," Ennis said. "I felt really emotional after the javelin because I knew it was so close.
"I just wanted to make sure I didnt get too carried away and finish the job off properly," said Ennis who sprinted away from the field down the home straight to win the 800 in a time of 2:08.65.
Going into the 800, Lithuania's Austra Skujyte was in second overall but she struggled around the two laps and crossed the line in last place, almost 12 seconds behind Ennis.
Ennis, 26, threw her arms aloft as she finished in a rare show of emotion after keeping her composure throughout the two-day competition.
Clearly struggling to hold herself together, she draped herself in a Union Jack flag and led the traditional joint lap of honour by all the heptathletes around a cheering stadium to the sound of David Bowie's 'We Could Be Heroes'.
After the misery of watching Nataliya Dobrynska win the Beijing heptathlon on television, Ennis came into the London Games in superb form and immediately made her mark on Friday with a world best of 12.54 for the 100 hurdles in the heptathlon.
She also set a personal best in the 200 on her way to her highest ever day-one total.
Ennis beat the three medallists from the Beijing Games - Dobrynska, who pulled out of the competition after a poor long jump, American Hyleas Fountain, who did not start the 800, and Chernova.
"I'm happy that I came here. It's amazing what's happening in this stadium. So many people helped us to compete," said Chernova, who had been expected to provide more of a challenge to Ennis.
"I was not in the best condition. I had a lot of problems here. I would do one good event and then another very bad."