Upon completion of his studies at the Conservatory of Liège, Léon Jongen became organist at the Saint Jacques church in his native city. In 1913, he won the First Grand Prize of Rome with his cantata Les fiancés de Noël. He started a career as pianist. In 1918 after World War I he travelled extensively to Africa, India, China and Japan and for 2 years was director and conductor of the Opéra Français of Hanoï.

Back in Belgium in 1934 he taught fugue at the Royal Conservatory of Brussels, afterwards he succeeded his brother Joseph as director of this institution. From 1939 to 1949 he conducted the concerts of the conservatory. His Violin Concerto was the compulsory work of the Queen Elisabeth Competition in 1963.

He wrote numerous symphonic works and he was attracted by the theatre. His opera Thomas l’Agnelet is one of the best lyrical works ever written in Belgium. Although a great admiror of the French romantic school and slightly influenced by César Franck he nevertheless developed towards more modernistic conceptions.