Simonaitis received his education in Heydekrug and Tilsit 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, 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, which adopted the Act of Tilsit but Simonaitis did not sign it. In 1919 he moved to Memel.
According to the Treaty of Versailles, the Klaipėda Region was detached from the German Empire and placed under provisional French administration. French General Dominique Joseph Odry established seven-member Directorate in February 1920. Initially all of its members were German. After Lithuanian protests, two Prussian Lithuanians, Simonaitis and Mikelis Reidys, were admitted to the Directorate increasing its size to nine members. Simonaitis resigned in February 1922. He lobbied the League of Nations and the Conference of Ambassadors to unite the Klaipėda region with Lithuania. However, others thought that the region should be turned into a free city, similar to the Free City of Danzig. Therefore, the Lithuanian government decided to organize the Klaipėda Revolt, take the region by force, and present a fait accompli. Simonaitis agreed to head the government, which would be installed by the rebels. On January 9, 1923, the Supreme Committee for the Salvation of Lithuania Minor declared that the old Directorate was dissolved and authorized Simonaitis to form a new Directorate within three days. The rebel's petitione to unite with Lithuania was approved by the First Seimas on January 14. To appease the Allies, Simonaitis was dismissed and replaced by Viktoras Gailius. After the Klaipėda Convention was signed in May 1924, formalizing the incorporation of the region into Lithuania, Simonaitis became chairman of the Directorate, governor of Šilutė and Klaipėda Counties, and mayor of Klaipėda.
In 1911 Simonaitis married a German woman, Margaretha Plaumann. They had four children: daughters Ana Marija and Birutė Neringa and sons Vitold-Vytautas and George-Jurgis. Vitoldas-Vytautas was mobilized into the Wehrmacht and died in 1941 in the Battle of Leningrad. Margaretha and Ana Marija attempted to flee Lithuania after the war, but were stopped by the Soviet police. They changed their names to Elena and Ona in an attempt to hide their German descent and returned to Klaipėda. Birutė Neringa was forcefully taken to Germany as a translator. She met her father at Dachau, but returned to Lithuania in 1948. His remains were reburied in Klaipėda cemetery in 1991.
