Kubiliūnas graduated from the Realgymnasium in Riga and continued his education at Vilnius Military School, graduating in December 1914. During World War I, he served in the Imperial Russian Army in the Eastern Front. He was assigned to the 2nd Riga Battalion of the Latvian Riflemen. He was awarded several Russian orders, including the Cross of St. George and promotion to captain for his actions during the Christmas Battles in 1916.