Riner was a member of the Levallois Sporting Club in Levallois-Perret, France and is coached by Christian Chaumont and Benoît Campargue. He won the World and European junior titles in 2006. In 2007, he won a gold medal at the European Judo Championships in Belgrade, Serbia, on the day after his eighteenth birthday. At the 2007 World Judo Championships in Rio de Janeiro, Brazil, he became the youngest ever senior world champion when he won the heavyweight (+100 kg) event, defeating the 2000 Olympic gold medallist, Kosei Inoue of Japan, in the semi-final.