Petronaitis was born on November 2, 1888, to a family of Petras Petronaitis, a well-to-do farmer, in Plauciškiai village, Rozalimas
Volost,
Ponevezhsky Uyezd,
Kovno Governorate. The village was at that time part of the
Russian Empire as a result of the
partition of the
Polish–Lithuanian Commonwealth in 1795. The family's circumstances were good and, after the graduation from the
Gymnasium of Mitava, he studied mathematics and science at the
Saint Petersburg State University. In
Saint Petersburg, he shared a house with his friend
Ignas Končius. In those days, many prominent Lithuanians studied in St. Petersburg, then the capital of the Russian Empire, including the future Lithuanian President
Antanas Smetona and Prime Minister
Augustinas Voldemaras. After graduating in 1913, he remained in Saint Petersburg and, at the outbreak of
World War I, was teaching mathematics. Like many young Lithuanians, he was drafted into the
Russian Army. In 1915, he was stationed in
Jaroslavl near
Moscow. Heavy Russian casualties in the war forced the Russian Army to set aside long-standing discrimination against Catholic Lithuanian soldiers. In 1916, Petronaitis was promoted to officer rank and became a teaching fellow at a Moscow military officers' school. At the same time he studied law at
Moscow University. == Army service and lawyer career==