Kasselstrand joined the Sweden Democrats while at university. He was chairman of the Sweden Democratic Youth from 2011 to 2015 and political secretary for the Sweden Democrats in the Swedish Riksdag from 1 January 2011 to 21 November 2012. He became acting chairman of the youth league upon the timeout of William Petzäll due to his alcohol problems in February 2011. He was elected ordinary chairman on 10 September 2011, and re-elected three consecutive times on 1 September 2012, 7 September 2013 and 29 November 2014. He resigned on the annual congress on 12 September 2015, having served as the chairman for nearly five years. In April 2015, Kasselstrand was expelled as a member from the mother party, the Sweden Democrats. The exclusion came after Kasselstrand's colleague and vice chairman of SDU, William Hahne, had been voted the chairman of the Sweden Democrat's Stockholm district, against the will of the mother party leadership. Kasselstrand called the action "undemocratic" and meant that it was all a way for the party leadership to secure their own power and positions, and that they feared the upcoming generation in the youth league to gain more influence. Kasselstrand continued on his position as the chairman for SDU until the ordinary congress in September the same year. After Kasselstrand resigned on the congress in September 2015, Jessica Ohlson won the election as the new chairman against Tobias Andersson, who had major support from the party leaders in his candidacy. Ohlson was considered to be one of Kasselstrand's closest collaborators, having served as the youth league secretary during Kasselstrand's chairmanship. Only hours after Ohlson was elected as the new chairperson, the Sweden Democrats announced that they broke all ties between the party and the youth league to form a new youth league. Meanwhile, Ohlson declared that she continue to lead the original youth league as an independent youth league. Kasselstrand and Ohlson publicly declared that they "will consider all opportunities" in the future regarding the possibility to form a new political party. Kasselstrand also called for party leaderJimmie Åkesson to resign from his position.
Alternative for Sweden
In January 2016, Kasselstrand announced that he had launched a podcast called "Den kokta grodan" with Erik Berglund. In March 2018, Kasselstrand and William Hahne launched a new nationalist political party called Alternative for Sweden, which they had registered in December 2017.
