Aho is one of Finland's leading symphonists, having written 11 works in that genre. He studied composition with noted symphonist Einojuhani Rautavaara at the Sibelius Academy in Helsinki and then later with composer Boris Blacher in Berlin (1971 - 1972).
Aho has since lectured at the University of Helsinki and served as a professor of composition at the Sibelius Academy. He is the composer-in-residence of the Lahti Symphony Orchestra. His earliest works include the string quartets written in 1967, 1970, and 1971, and a violin sonata of 1973. His first four symphonies (1969 - 1973) employ classical polyphony and have timbres suggesting the influence of Dmitry Shostakovich and Gustav Mahler. Symphony No. 11 was premiered by the Lahti Symphony Orchestra and the Kroumata Ensemble under the direction of Osmo Vänskä in March 2000. The orchestral work entitled The Rejoicing of the Deep Waters was premiered in September 1998. His works for the stage include a dramatic monologue for singer and chamber orchestra, entitled Avain/The Key, and his two-act opera Hyönteiselämää/Insect Life (1996). Aho's other works include concertos for violin, piano, and cello; various solo pieces; as well as orchestrations of other composers' works, such as his successful version of Modest Mussorgsky's Songs and Dances of Death. Aho is also a noted writer.