Mäntyjärvi was born in Turku. He studied musicology, English philology and linguistics at the University of Helsinki, graduating with an FK (master's) degree in 1991. In 1987, he was accredited as an Authorized Translator from Finnish to English and English to Finnish, and since then he has been employed as a professional translator. He has also studied theory of music and choir conducting at the Sibelius Academy.
As a composer, Jaakko Mäntyjärvi describes himself as an eclectic traditionalist: eclectic in that he adopts influences from a number of styles and periods, fusing them into his own idiom; traditionalist in that his musical language is based on a traditional approach and uses the resources of modern music only sparingly. Because he is himself active in making music, his music is very practically oriented; he is a choral singer, and thus most of his works are for choir.
His major choral works include Four Shakespeare Songs (1984), Dagen svalnar... (Day is cooling; 1991/93), Ave Maria (1991), Pseudo-Yoik (1994), El Hambo (1997), and More Shakespeare Songs (1997); his work Canticum Calamitatis Maritimae received third prize in the European composition competition for cathedral choirs in 1997. His major commissions include a work for mixed choir for the contemporary choral music seminar at the Cork International Choral Festival in spring 1999, a choral work for the 700th anniversary of the consecration of Turku Cathedral in summer 2000, and commissions from Chanticleer (2001) and the King's Singers (2002). He was composer-in-residence of the Tapiola Chamber Choir from 2000 to 2005. In 2006 he was commissioned to write a new 40-part work for choir, Tentatio, for the Tallis Festival.
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. He conducted the Savolaisen Osakunnan Laulajat student choir from 1988 to 1993 and was deputy conductor of the Tapiola Chamber Choir from 1998 to 2004.
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.
In 2012 he was commissioned by Exultate Singers based in Bristol, UK to write a setting of Iste Mundus, premiered at St George's Bristol on Saturday 23 February 2013.