He was born in Kaunas, Lithuania, the son of Jokūbas Šernas, a signatory of the 1918 Act of Independence of Lithuania, who died when Sernas was one year old. His mother took him to Paris, France, where Sernas received his formal education. During World War II, he joined the French Resistance and was captured and interned at the Nazi Buchenwald concentration camp, near Weimar, Germany. After the war, Sernas was considering a career in Medicine and then in journalism before turning to films. His first role was in Miroir (1946).