In 1950, he started his professional music career as a promising trumpet player at Radio Ljubljana Dance Orchestra. In 1956, Sepe married Slovenian singer Majda Sepe. He also established the jazz Mojmir Sepe Band (Ansambel Mojmirja Sepeta), which published the first jazz vinyl record in Yugoslavia. His trumpet career ended after an altercation at Opatija Festival '65 when four guys knocked out a couple of his front teeth as he defended her from having her purse stolen. He committed to composing and conducting, mostly influenced by jazz and swing music.