In 1990, Tekin, together with his school friend Kerem Kabadayı founded a group «Decision». Other members were Şahin Yalabık (vocals), Alper Tekin (bass guitar). When in 1991 the group moved to a rock sound, Harun started playing the guitar. Later, they were joined by Derin Esmer (guitar). After Yalabık left the group, the musicians decided to move to Turkish lyrics, and founded in 1995 the group of Mor ve Ötesi («Violet and ultra»). In the new group, Tekin shared the role of soloist with Derin Esmer, but after the latter's retirement in 1999, Tekin became the only vocalist. He and Burak Güven are the main writers of the group.
In 2007, he performed the soundtrack for the film director Veli Çelik's Kayıp. In 2008, he performed a song "İstasyon İnsanları" from the album of Teoman Söz Müzik Teoman.