Mieczysław was born in his family manor in Saldutiškis (Syłgudyszki) on the lands of the partitioned Polish–Lithuanian Commonwealth (then in the Russian Empire, now in Lithuania), or in Rostov-on-Don. His mother, Anela, was a sister of Stanisław Witkiewicz; she was also a distant relative of the Piłsudski family. His father, general Bolesław, was an engineer, the grandson of a Russian Army general who sided with the Polish 1830–31 insurgents during the November Uprising and was sentenced to death, but pardoned.