[2] 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 1962 he became a member of the Finnish Academy of Science and Letters (Suomalainen Tiedeakatemia).
[6] In 1975 he was given by the President of Finland the honorary title "Academician of Science" (Tieteen akateemikko).
Lehto was the chief organizer of the International Congress of Mathematicians (ICM) in Helsinki in 1978 and an invited speaker of the ICM in Moscow in 1966 with lecture Quasiconformal mappings in the plane.