The 20-year-old’s late surge saw her pass longtime leader and defending champion Olga Kaniskina of Russia (1hr 25min 09sec) while China's Qieyang Shenjie set an Asian record in taking bronze (1hr 25min 16sec).
It was the first time Russia have taken gold and silver in the event and third time in six editions they have won the title.
Lashmanova’s victory had looked a longshot as Kaniskina dominated until she tired dreadfully on the last two laps and had nothing left as the teenager swept past her with just 50 metres to the line to go.
Kaniskina, 27, burst into floods of tears after a consoling hug from the winner. Kaniskina had broken away in the early stages accompanied solely by China's two-time world medallist Liu Hong.
However, Liu was dropped by Kaniskina and was sandwiched in between her and a group of five including the other two Russians and Guatemalan Mirna Ortiz.