Compatriot and 2008 champion Olga Kaniskina took the silver seven second behind after being overtaken by Lashmanova in the final few metres having led from the start.
China's Qieyang Shenjie was third for the bronze medal.
Three-times world champion Kaniskina had been aiming to become the first woman to retain the 20-km walk title but crossed the line in obvious pain and was limping as she congratulated her younger team mate.
Russia's Sergey Kirdyapkin won gold in the men's 50km race walk in an Olympic record time of three hours, 35 minutes and 59 seconds.
The 32-year-old double world champion shaved more than a second off Alex Schwazer's record of 3:37:09 to finish 54 seconds ahead of Australia's Jared Tallent, whose time was also inside the previous Olympic record and a personal best.
"Around the 25-30km mark I hit the wall. I had to fight with myself but I found my second wind. If I didn't have that small problem I would have done a better time," said Kirdyapkin, who did not finish the 50km event in Beijing four years ago.
China's Si Tianfeng, 28, won bronze in a personal best of 3:37:16, adding to Chen Ding's 20km gold and Wang Zhen's bronze a week ago - China's first men's race walk medals.
The Russian trio of Kirdyapkin, Igor Erokhin and last year's world champion Sergey Bakulin were strong medal favourites after Italy's defending champion Schwazer was excluded from the London Games on Monday for testing positive for EPO.
They were front runners from the start of the 25-lap race, which passed London's Buckingham Palace, and with 10km to go were leading after reeling in Si who had broken away to an 18-second lead.
Si's injection of pace into the race had tired both Erokhin and Bakulin, though, and with another push the Chinese moved back into second behind a determined-looking Kirdyapkin, who kept up a solid pace in the sunshine.
With 30 minutes to go before the finish Tallent, whose wife Claire will compete in the women's 20km race walk on Saturday, made his move on Si to take second but was unable to get close to the Russian leader.
It was Tallent's second consecutive Olympic silver medal in the event and adds to the 20km bronze he also won in Beijing.
"I gave it all I could in the last 10km, I came home as hard as I could but I just couldn't catch Kirdyapkin, he's a true champion. He's won two world championships, one world cup and now the Olympic gold medal," the 27-year-old said.
"I don't think I could have done anything else. I came home as hard as I could, 42 minutes for the last 10km, I don't think I could have done much more."
Before Saturday's race Kirdyapkin had already set this year's leading time (3:38:08) when he won the 2012 World Race Walking Cup in May.
Ireland's Robert Heffernan set a national record time to finish fourth ahead of Erokhin and Bakulin after an impressive late surge through the field.