Janez Lenassi was born in Opatija in Croatia. He studied sculpture under Professors Boris Kalin, Frančišek Smerdu, Zdenko Kalin, Karel Putrih and Peter Loboda at the Academy of Fine Arts in Ljubljana. He graduated in 1951. He continued his studies with a specialization in sculpture under Professors Frančišek Smerdu and Boris Kalin, and completed it in 1956.
In 1966, He won the Prešeren Fund Award for his sculpture in the complex of the National Liberation War Monument in Ilirska Bistrica.