In 2015, she won gold in the all-around, ribbon final and hoop final at the Moscow Grand Prix. She also won bronze in the ball final. At the 2015 European Championships, Mamun and her teammates won team gold. She won gold in the hoop final and silver in the ball final. In 2015, the European Games were held for the first time. Mamun competed at the European Games and won the all-around silver and a gold medal in the hoop final. At the 2015 World Championships, Mamun and her teammates won team gold. Mamun won a gold medal and three silver medals. She was awarded the Longines Prize for Elegance. Mamun was ranked second in the world in 2015.