Lesun started out his sporting career as a swimmer, before switching to modern pentathlon at the very young age. He first competed at the European and World Junior Championships in 2008, and had achieved five top-ten finishes. In late 2009, Lesun moved to Russia from Belarus, and obtained a dual citizenship and a Russian passport. After having missed the 2009 breakthrough season in modern pentathlon because of citizenship issues, Lesun returned to the international scene in early 2010, and was formally admitted to the national team. He competed at the World Championships in Chengdu, China, where he won two medals, one for the individual and one for the team relay. He also added his first ever gold medal at the 2010 World Cup in Budapest, Hungary. He also led a strong Russian team, including former champion Ilia Frolov and two-time Olympic championAndrey Moiseev to the gold medal at the World Championships in Moscow, in addition to his silver in the individual event. In 2012, Lesun emerged as Russia's best medal prospect, and a top favorite to win gold at his first Olympics. He had broken his streak of silver medal finishes in his 2011 breakthrough season and also beat the former world and defending Olympic champion Moiseev to win his first individual gold medal at the 2012 World Modern Pentathlon Championships in Rome, Italy. Following his triumph, Lesun qualified for the 2012 Summer Olympics in London, along with Moiseev, to compete in the men's event, which included a first-ever combined running and shooting segment. During the competition, Lesun made a strong showing in the fencing segment, tying in second place with China's Cao Zhongrong for a score of twenty-five victories, but struggled to maintain his position in the entire event, with disappointing scores in freestyle swimming and horse riding. At the combined running and shooting discipline, Lesun progressed into the event in third place, until he was surpassed by Hungary's Ádám Marosi to win the bronze medal, dropping him out of the podium to a fourth-place finish, with a score of 5,764 points. In 2016, he won the gold medal at the Rio Olympics with a total of 1479 points. At the 2017 World Modern Pentathlon Championships in Cairo, Egypt, Lesun suffered a back injury in the fencing sports, leaving him without a medal in the individual competition. However, he won bronze with his team.
