Komova was reduced to tears for the second time in three days when American Douglas pipped her to the title by 0.259 of a point after producing the day's best performances on the vault and the beam.
Aliya Mustafina, who with Komova was disappointed to take team silver behind the Americans on Tuesday, clung on for bronze despite a fall from the beam. She and Douglas's compatriot Aly Raisman finished with the same total but the Russian won the medal on the tiebreak rule.
Douglas, dubbed the "Flying Squirrel" for the shape she produces on the bars, was watched from the stands by team mate and world champion Jordyn Wieber, who had come into the Games touted as the favourite for Thursday's honours but failed to qualify for the final.
Rules and regulations seem to be plaguing the Americans here. Wieber finished fourth in qualifying for the all-around but missed the cut since each nation is allowed only two women in the final. As Douglas and Raisman ranked above her in the preliminaries, Wieber was demoted to the role of spectator at the North Greenwich Arena on Thursday.
Raisman lost out on bronze despite finishing off with the second-best floor routine of the evening under the rule which separates equally-placed contestants by totting up the totals of their three best apparatus.
Douglas, though, was beyond the reach of such concerns, leading from the first of the four rotations when she was the opener on the vault.
A slight hop sideways on landing could have cost her but all her rivals fluffed their landings, with Komova stumbling sideways right off the mat.
Raisman banged her foot on one of the asymmetric bars in the second routine and began to look concerned. Douglas, for all her prowess on the apparatus, was beaten by the two Russians, with Mustafina scoring a high 16.100, but the American stayed in the lead.
With Douglas and the 17-year-old Komova duelling for the gold, their team mates were left to fight for bronze and Mustafina looked to have thrown away her chances when she came off the beam attempting to land a twisting somersault.
Her score was a low 13.633 and Raisman took to the narrow piece of wood knowing she could take advantage. Her hopes shrivelled, though, when she only just saved herself from overbalancing and then wobbled precariously on a spin and she dropped to fifth place.
Though she recovered with 15.133 on the floor, where she won a world bronze medal last October, it was not enough to put her ahead of 2010 world all-around champion Mustafina.
Komova was last on the floor and Raisman stood with her arm around Douglas as they waited for the giant scoreboard, high above them, to show their fate. Seconds later, only Douglas was celebrating.
As Raisman bit her lip and Komova slumped in a chair and covered her face with both hands, Douglas climbed on to the dais by the vault run-up and waved to the wildly cheering and flag-waving American fans.
Her victory was another feather in the cap of Chinese-born coach Liang Chow who coached another American, Shawn Johnson, to all-around silver and beam gold at the Beijing Olympics four years ago.
"It just feels amazing to be called the Olympic champion," Douglas told a news conference, chattering at high speed. "I am on cloud nine."
Douglas said she thrived on the pressure of big competition. "It helps me and motivates me," she said. "It definitely pumps you up and you want to stick every landing and do sharp movements."
Asked if she had stayed focused or had tracked the scores of her rivals, Douglas admitted to having "a quick peek" at the scoreboard, prompting Chow, who was sitting beside her, to laugh and say: "She broke the rules."
So when had she had this quick peek, a journalist asked. Douglas smiled. "I looked up after the vault... and the bars... and the beam... and the floor," she admitted, prompting more chuckling from her coach of nearly three years.
Komova, by contrast, cut a dejected figure despite helping Russia to get their best all-around showing as an independent nation, and was unimpressed with the colour of her medal, putting it in her pocket before she faced reporters waiting to talk to her.
"It was too heavy," she said through a translator. "I am still upset because it could have been gold."
- Aly Raisman