Uniłowski was born in Warsaw in a petty-bourgeois family. His parents were Władysław Uniłowski and Jadwiga Uniłowska née Ciechonski. After the death of both parents (his mother died of tuberculosis, while father committed suicide), as a teenage boy, he lived with his aunt, a poor seamstress in Powiśle. Not completing junior high school for financial reasons, he was forced to take up employment, including as a bricklaying helper, a parcel delivery service for a tailor or a waiter boy in the Warsaw restaurant "Astoria". There, in 1926, Karol Szymanowski noticed him, being intrigued by the young waiter who read Joseph Conrad's prose. Szymanowski's financial assistance enabled Uniłowski to self-study, facilitated literary contacts, and above all, enabled him to stay frequently in Zakopane, which was advisable due to developing tuberculosis.
