He attended Children and Youth Sports School of Sovetsky District, Khanty–Mansi Autonomous Okrug, where his first trainer was Valery Ukhov.

Dementyev finished 0.8 seconds behind the winner Giorgio Di Centa in the 50 km event, the closest margin of victory in Olympic history of this event.

This margin of victory eclipsed the previous record of 4.9 seconds set at the 1984 Winter Olympics in Sarajevo between fellow Swedes Thomas Wassberg and Gunde Svan.

Dementyev has two 4 × 10 km medals at the FIS Nordic World Ski Championships with a silver in 2007 and a bronze in 2005.

On 25 August 2009, Dementyev tested positive for recombinant erythropoietin (EPO).