Leif Segerstam is primarily known as a conductor, generally ranked among the most prominent from Finland in the latter 20th and early 21st centuries. However, he is also a composer with a massive output, making the cataloging of his music difficult. For instance, by October of 2021, Segerstam had written 334 symphonies, 30 of them having been composed or completed in 2005 alone. Yet that seemingly enormous body of music is not quite what it seems, since he typically writes the same piece in different scorings using aleatoric methods involving less-complex and less-specific notation. Still, his output contains countless orchestral and choral works, concertos, songs, string quartets, and instrumental pieces. In 2020, Segerstam led the Turku Philharmonic in a recording of Mahler's Symphony No. 4 on the Alba label.

Segerstam was born in Vaasa, Finland, on March 2, 1944. He entered the Sibelius Academy of Music in Helsinki in his mid-teens, where he studied violin with Liisa Siikonen and conducting with Jussi Jalas. Segerstam was also an excellent pianist then and won first prize in the 1962 Maj Lind Piano Competition, held by the Sibelius Academy. The following year, he graduated with degrees in both violin and conducting and gave his first major concert as a violinist. By then, he had already written his earliest compositions, which included the brief 1960 orchestral work A Legend (Nils-Eric Fougstedt in Memoriam) and Three Songs for soprano and piano (1960-1961). Following Segerstam's graduation, he enrolled at the Juilliard School of Music, where he studied violin with Louis Persinger, conducting with Jean Morel, and composition with Vincent Persichetti and Hall Overton.

In 1965, Segerstam received his first important conducting appointment to the Finnish National Opera as director for the 1973-1974 season. Segerstam held two other conducting posts at opera houses in Scandinavia and Germany, at the Royal Opera in Stockholm from 1968 to 1972, and at the Deutsche Oper in Berlin for a single season (1972-1973). Segerstam turned out many compositions during these early conducting years, including several string quartets (the Fifth, subtitled "Lemming," was a stylistically pivotal work in his use of aleatory methods) and the song collection Tre plus eller fyra NNNUUUU-R, for mezzo-soprano and piano, on texts by Gunnar Björling.

Segerstam successively took three conducting posts with second-tier European orchestras in the period from 1977 to 1985 (the Austrian Radio Symphony Orchestra, the Finnish Radio Symphony Orchestra, and the Rheinland-Pfalz State Philharmonic Orchestra). He was appointed chief conductor of the Danish National Radio Symphony Orchestra in 1989. For the labels BIS, Ondine, and Finlandia, he turned out performances of his symphonies No. 9, No. 11, No. 13, among many others, and violin concertos, Concerto Serioso and A Last Melodioso, with his then-wife Hannele Segerstam as soloist. Upon his departure from the Danish National Radio Symphony in 1995, he accepted the position of chief conductor for the Helsinki Philharmonic Orchestra and returned to Stockholm's Royal Opera, once more as music director. Segerstam remained with the Royal Opera until 2000 and stepped down from his post with the Helsinki Philharmonic in 2007, taking the title of chief conductor emeritus. At the Royal Opera House, Segerstam has conducted a wide range of works. In 2002, for example, he led performances of Wagner's Tristan und Isolde, Prokofiev's The Fiery Angel, and Janácek's The Makropulos Affair. He has continued to conduct and compose into the 2020s, penning no fewer than five symphonies in 2021. Among the orchestras he has guest conducted are the Chicago Symphony, the Los Angeles Philharmonic, and the Turku Philharmonic. Segerstam led the latter group in a recording of Mahler's Symphony No. 4 on the Alba label in 2020. ~ Robert Cummings & Keith Finke, Rovi