The 38-year-old took time off after the Beijing Games to start a family but could not resist the urge to get on the bike again, coming into the Olympics hoping to give her son Lucas "something to play with".
The double time-trial world champion produced a steady performance over the 29-kilometre course to earn a seat on one of the three thrones installed to welcome the leaders near the finish line, before receiving her medal in the forecourt of Hampton Court Palace with tears in her eyes.
Lucas was there and walked away with his mother and a bouquet in his hands before getting hold of the medal.
"Today I didn't know until I crossed the finish line. People try to tell you are on track, but out on the course today, the information I was getting was that it was a close race. I just needed to give it everything if I really wanted it," said Armstrong, who like silver medallist Judith Arndt will definitely not be at the Rio Games in 2016.
"I'm sure I won't race in Rio, I'd rather come back as a mentor," she told Reuters.
"The last 20 months have been a roller coaster. It was not easy. I want to be an inspiration for all the moms out there."
German Arndt, 36, told Reuters: "I always hope. When you race you want to win but yes I was kind of expecting Kristin to win.
"I won't be in Rio either. I don't want to suffer anymore."
Armstrong averaged over 46 kph on the 29-kilometre course to beat world champion Arndt who took silver 15 seconds behind, according to provisional results.
Russian Olga Zabelinskaya, the road race bronze medallist, claimed bronze again, 22 seconds off the pace.
"Two medals were a dream I wasn't expecting. Tonight no celebration, I'm going home in the morning," she told reporters.
On a course setting off and finishing at the opulent Hampton Court Palace, built by King Henry VIII on the banks of the Thames on the outskirts of London, Armstrong made the best start.
She clocked the fastest time at the first check point after 9.1 kilometres on Burwood road ahead of New Zealand's Linda Melanie Villumsen and Canadian Clara Hughes under threatening skies.
On dry roads, Arndt made a strong impression as she powered past road race Olympic champion Marianne Vos of the Netherlands, who left the start house 1:30 earlier, before the halfway point.
At the second check point after 20.1 km, Arndt was third, eight seconds off Armstrong's pace and eventually had to settle for silver.
British hopeful Emma Pooley took sixth place, with road race silver medallist Lizzie Armitstead finishing 10th and Vos 16th.
"You always have to push your hardest and I did. I don't think I could have gone any faster. In time trial you can't affect anyone else,” Pooley said.
“I'm really disappointed but if someone else is faster than you, you can't do anything.
"I was really pleased to help Lizzie in the road race. I just wish we could have gone up Box Hill in the time trial."
Armitstead added: "That was just awful. You have just got to put yourself in such a dark, dark place to get through that with your mind. When I saw the 10 km to go banner, I was so happy. I've never felt such pain like that."
"It didn't feel like a lap of honour [after her earlier road race silver], the whole thing was just painful. It is something I need to improve on if I want to compete in it
“I'm disappointed for Emma after all the work she put in and helped me in the road race."