Simonaitis received his education in Heydekrug (Šilutė) and Tilsit (Sovetsk) and worked as a court clerk. He became a Lithuanian activist in 1909. In 1912 he was co-founder of the Lithuanian Club of Tilsit (Lithuanian: Tilžės lietuvių klubas), which he chaired for two years. In 1915, Simonaitis was mobilized into the German Army. He was awarded the Iron Cross for his military service in France and Galicia. After World War I he briefly worked in Vilnius where he became acquainted with Lithuanian politicians and the Council of Lithuania. After returning to Tilsit in 1918, he participated in the activities of the
National Council of Lithuania Minor (Lithuanian: Mažosios Lietuvos tautinė taryba), which adopted the Act of Tilsit but Simonaitis did not sign it. In 1919 he moved to Memel (Klaipėda).