Born in Forssa, Finland, he studied composition at the Sibelius Academy under Einojuhani Rautavaara, receiving a diploma in 1971. He continued his studies for a year in Berlin with Boris Blacher. His teaching positions include music theory at the University of Helsinki from 1974 to 1988, and a professorship at the Sibelius Academy from 1988 to 1993. He became composer-in-residence for the Lahti Symphony Orchestra in 1992, and conductor Osmo Vänskä has recorded many of his recent large-scale works with the orchestra. Aho has worked as a freelance composer, with a state scholarship, since 1993. He lives in Helsinki.
Known principally as a composer of large-scale works, to date Aho has composed sixteen symphonies, twenty-four concertos, five operas and several vocal works. His chamber music includes several quintets, quartets, sonatas and solo works. He first came to fame with his first symphony (1969) and second string quartet (1970). His works of this time showed such neo-classical traits as a preoccupation with counterpoint (particularly fugues), and stylized renderings of older forms, such as the waltz. In the following decade he wrote in modernist and post-modernist styles. His use of irony and juxtaposition of contrasting moods and musical styles and genres has been compared to Gustav Mahler and Alfred Schnittke. His work also shows the influence of Einojuhani Rautavaara (especially when evoking mysterious textures) and Dmitri Shostakovich. His music has been extensively recorded by BIS records and is published by Fennica Gehrman.