He studied musicology, English language and literature and linguistics at the University of Helsinki, graduating with the equivalent of an MA degree in 1991.
Mäntyjärvi is a professional freelance translator and composer, and also an amateur and semi-professional choral singer and conductor.
As a composer, Jaakko Mäntyjärvi describes himself as an eclectic traditionalist: adopting influences from a number of styles and periods, and basing his musical idiom largely on traditionally oriented materials.
His major choral works include Four Shakespeare Songs (1984), Dagen svalnar... (Day is cooling; 1991/93), Pseudo-Yoik (1994), El Hambo (1997), the 40-part motet Tentatio (2006), Stuttgarter Psalmen (2009) and the Trinity Service (2019).
His work Canticum Calamitatis Maritimae received third prize in the European composition competition for cathedral choirs in 1997.