Born in Liptovská Porúbka, Slovakia, he graduated from a chemistry high school and worked for the Baťa company.

Lenart was a member of the federal parliament (whose name changed several times) from 1960 to 1990, and was Speaker of the Slovak National Council from 1962 to 1963.

On the basis of insufficient evidence, on 23 September 2002 Lenárt was acquitted of treason charges (along with his co-defendant Miloš Jakeš), related to his handling (or lack thereof) of the Prague Spring events in 1968.

[4] He was accused of attending a meeting at the Soviet embassy in Prague on the day after the 1968 Warsaw Pact invasion, planning to establish a new "workers and farmers'" government.

Jozef Lenárt was one of the most resilient figures in Czechoslovakia's communist hierarchy, occupying one post or another in the leadership for no less than a quarter of the century.