Leonas was born on 15 January 1894 to a family of Lithuanian farmers. After graduation from the Second Men's Gymnasium in Vilnius, he continued to study at the Moscow University and Kharkiv University but was drafted to the Russian Imperial Army in May 1916. He completed military courses in Chuhuiv and was promoted to praporshchik in September 1916. After the promotion to podporuchik in July 1917, Leonas was transferred to the 2nd Battalion of the Russian Railway Troops. Leonas returned to Lithuania in 1919 and was mobilized on 1 July 1919 into the Lithuanian Army and assigned to the. He saw action in the Lithuanian Wars of Independence against Soviet Russia and Poland. In February 1920, he helped to subdue a revolt by Lithuanian soldiers in Kaunas and was promoted to captain. He completed Higher Officers' Courses in October 1923 and was promoted to major. He retired from the military after graduating from the Law Faculty of University of Lithuania in 1925. He worked as a judge in Biržai, Telšiai, Šiauliai. After a judicial reform in 1933, he became a judge of the appellate court. In June 1934, he returned to the military duty and became the presiding judge of the Military Tribunal. He presided over the high-profile trial of Neumann–Sass. The case involved 122 defendants – Nazi sympathizers that were accused of various anti-Lithuanian activities in the Klaipėda Region. In March 1938, after the Polish ultimatum to Lithuania, he became Minister of Internal Affairs in the new cabinet assembled by Prime Minister Vladas Mironas. In this capacity, Leonas issued many laws and directives, many of them on sale of pharmaceuticals, health clinics, and other health care issues. He also implemented the law establishing a council to monitor and control various societies and organizations giving even more power to the authoritarian regime of Antanas Smetona to control political opponents. The government was forced to resign after the German ultimatum to Lithuania in March 1939. He again retired from the military on 31 March 1939. In May 1940, Leonas was appointed as a judge of the Lithuanian Tribunal. After the Soviet occupation of Lithuania in June 1940, Leonas was arrested on 11 July 1940 and imprisoned in Kaunas and Lubyanka Prisons. He was freed at the start of the German invasion of the Soviet Union and anti-Soviet uprising in June 1941. During the German occupation, Leonas worked as an attorney in Kaunas. When the Red Army returned to Lithuania, Leonas was arrested by the NKVD in November 1944 and sentenced to 10 years of prison and 5 years of exile. He was imprisoned in a Gulag camp in Abez, Komi Republic. Later he was transferred to the Irkutsk Oblast. He returned to Lithuania in 1956 and died three years later in Kaunas.
