Grandelius came under the limelight after winning several notable championships. He emerged victorious in the 2008 Swedish Chess Championship despite being the youngest participant. The same year, he also won the World Youth Chess Championship for under-16 boys. In 2010, he became the youngest-ever Swedish champion and continued to defend his title in 2011, 2017 and 2018.
Proving his international prowess, Grandelius won the Politiken Cup in Copenhagen in 2013 and the Sigeman Chess Tournament in Malmö in 2015. He also demonstrated strong performances in team events. As a part of the Swedish national team, he participated in multiple Chess Olympiads. His expertise also earned him a spot in several World Team Chess Championships and European Team Chess Championships.