Mryj was born into a middle-class family in the Mogilev Governorate of the Russian Empire (nowadays the Mahilioŭ region in eastern Belarus)..  In 1914, he graduated from a seminary in Mahilioŭ and continued his theological education in Kyiv.  In 1916 he was conscripted into the Russian Imperial Army. After the Bolshevik revolution, he served in the Red Army