Mamertas Čepulis was born in Žuraviškės village, Mielagėnai rural district, Švenčionys county. In 1937 he started a primary school in Mielagėnai, finished it during the years of the German occupation and entered Vilnius 1st gymnasium for boys. Difficult living conditions coupled with family problems forced Mamertas to continue education at Švenčionys gymnasium. There was an episode in his life when he was in the last form of the school and at the same time worked at the meteorological station of Švenčionys to help his ailing father. Despite such stressful conditions, he finished the gymnasium successfully and became a student at the Faculty of Law of Vilnius University. Unfortunately, his studies were disrupted by Soviet repression: in 1949, Mamertas was deported to Irkutsk region during a mass deportation operation. He was released in 1957 and returned to Lithuania with his parents. In 1961 he filed a request with Vilnius University asking for his school-leaving certificate to be released to him from the university archives. Presumably he intended to study at another institution of higher learning.