Mäntyjärvi has been active as an amateur and semi-professional musician, mainly as a choral singer with a number of Finnish choirs, including the Savonlinna Opera Festival Choir, the professional Sibelius Academy Vocal Ensemble and the Tapiola Chamber Choir.
In 2011 Mäntyjärvi composed a choral work Cornish Lullaby for Helsinki Academic Male Choir KYL (Kauppakorkeakoulun Ylioppilaskunnan Laulajat), which was one of the pieces of the winning performance in the V Cornwall International Male Voice Choral Festival.