Hemar was born Jan Marian Hescheles in an assimilated Jewish family, the son of Ignacy and Berta neé Lem (Stanisław Lem, one of Poland's most famous writers of all time, was Hemar's cousin on his mother's side of the family). He lived in Lviv for the first 24 years of his life, then moved to Warsaw for 14 years until ending up in London, where he stayed for 30 years until his death. Hemar studied medicine and philosophy at the Jan Kazimierz University in Lviv. His poetic debut, the piece Więzień (Prisoner, trans. NS), was published in Gazeta Poranna i Wieczorna (no. 4771).
At 17, Hemar joined the army as a volunteer and later fought in the Polish-Soviet War of 1920. Once combat was over, he went back to study but never graduated from university. Instead, Hemar started co-operating with Henryk Zdzierzchowski, the editor of the satirical magazine Szczutka, publishing his works in the periodical: epigrams, fairy tales, poems, sketches and parodies. In the early 1920s, Hemar also managed to publish two books: the collection Dzik i Świnia (Pig & Boar, trans. NS, 1922) and Ballada o Białym Byku (The Ballad of a White Bull, trans. NS, 1923).
The revue Łątki Lwowskie (The Damselflies of Lviv, trans. NS), written chiefly by Hemart and staged in 1924 in Miejski Theatre, turned out to be a success. Thanks to Ludwik Ławińsi and L. Orlański, in December 1924 a selection of Hemar's best works was featured in Qui pro Quo – the legendary revue theatre of the interwar period. While in Lviv, Hemart also had a brief period of co-operation with Bruno Jasieński, the famous Futurist writer. The two established a short-lived cabaret; Jasieński soon left for Paris as the correspondent of Wiek Nowy and Hemar was invited to co-operate with Qui pro Quo on a regular basis.