Armstrong covered the slippery 29.7km course in 44 minutes and 26 seconds, six seconds faster than Russia's Olga Zabelinskaya and 11 seconds in front of 2016 Olympic road race champion, Anna Van der Breggen (Netherlands).
The American was last down the ramp in the wet and blustery conditions, hitting the first check point (at 10km) with the best time, five seconds ahead of Elisa Longo Borghini (Italy), and seven from Van der Breggen.
Here, Zabelinskaya was 23 seconds down but she dragged it back by the second check point and the finish line where she posted the fastest times.
Two seconds behind the Russian by the second check point, the 42-year-old brought it home fast to win the gold by a relatively small margin.
"It's amazing, I don't know if it has hit me yet," Armstrong said post race.
"This has been the most difficult journey. I took time off. The mind gets tired. So when I came back I was focused on Rio and it's been difficult because I've had some poor performances over the last six months." "When I looked out of the window at 4am I thought, 'Oh!' But I had two choices, get nervous, or say I'm the most experienced person out here and just attack it."
To climb the top step in Rio after London 2012, Armstrong triumphed over three hip replacements, osteoarthritis, a second retirement comeback, and an ordinary last six months of TT form.
"Half the battle is mental ability, as long as my mind is there. You can do anything you want if you put your mind to it," she said.
Australia's Katrin Garfoot raced impressively finishing ninth on Olympic TT debut. The 34-year-old suffered a training setback when she came down with a chest infection.
"I've had four weeks to prepare myself. I am very happy especially considering the (wet) conditions," Garfoot said.
"I wasn't sure how hard to go in the first part because of all the up and down hills and the wet so I didn't want to risk too much but then I think the others would've too.
"In the end I had a lot left so I pushed it back hard home which mightn't have helped on the flats. But considering everything I was really happy with my ride and my first Olympic Games."
Katrin Garfoot (Australia) finished ninth on Olympics debut (Getty) Source: Getty Images
Controversy
Zabelinskaya's podium finish was far from popular in the cycling and sporting world. Brit Emma Pooley stated previously she would refuse to shake her hand and Garfoot after today's race said karma often sorted things out in the sporting world.
Like the UK's Lizzie Armitstead, the Russian won a last minute reprieve from the Court of Arbitration of Sport (CAS) to participate in Rio.
Zabelinskaya, who served a retroactive doping ban for the banned substance octopamine, formed part of the IOC's blanket ban on all Russian convicted dopers participating at Rio. CAS ruled this unenforceable and Zabelinskaya proceeded to Rio while she awaited the outcome of a tranche of appeals. She had booked her ticket to Russia just in case.
“5th of August I had my ticket to go to Russia, they tell me I stay here and I was on the way to the airport,” she said.
“In the morning they told us we participate, in the evening they say that we don’t. It was two weeks like this, in the morning we go, in the evening we don’t.”
But this wasn't the only controversy of the race with many still questioning the manner of Armstrong's Olympic selection.
Although Armstrong performed impressively in 2015 post retirement, her form in the last six months has been questionable. She was defeated by compatriot Carmen Small in the Cascade Classic TT just three weeks ago, but also when it mattered the most at the national championships; arguably the USA Olympic team's selection event.
What might've been
Dutch TT powerhouse Ellen van Dijk flew home to a fourth place just 22 seconds off the pace. Around the 10km mark however, she overcooked a corner on a climb, riding off the road. The delay to her time and rhythm definitely cost her a medal.
It was a similar situation for Ashleigh Moolman Pasio. The South African was on a flyer at the first check point in seventh and progressively improving. But a crash cost her a strong finish.
