Lehto earned his PhD in 1949 from the University of Helsinki under
Rolf Nevanlinna with thesis Anwendung orthogonaler Systeme auf gewisse funktionentheoretische Extremal- und Abbildungsprobleme.[3] At the University of Helsinki, Lehto was from 1961 to 1988 a professor, from 1978 the dean of science, from 1983 the
rector, and from 1988 to 1993 the chancellor.
In 2001 Lehto published a biography of his mentor Rolf Nevanlinna.[7]
Selected works
with Kaarlo Virtanen: Quasikonforme Abbildungen. Springer 1965, Grundlehren der Mathematischen Wissenschaften, 2nd edition: Quasiconformal mappings in the plane. Springer 1973.
Univalent functions and Teichmüller Spaces. Springer, Graduate Texts in Mathematics, 1987.[8]
Mathematics without borders: a history of the International Mathematical Union. Verlag-Springer. 1998.
ISBN978-0-387-98358-5.[9]
Lehto, Olli (2001). Korkeat maailmat: Rolf Nevanlinnan elämä [High Worlds: The life of Rolf Nevanlinna] (in Finnish).
Otava.
OCLC58345155.