Słonimski was the grandson of Hayyim Selig Slonimski, the founder of "ha-Tsefirah"- the first Hebrew weekly with an emphasis on the sciences.

In 1919 he co-founded the Skamander group of experimental poets with Julian Tuwim and Jarosław Iwaszkiewicz.

Słonimski spent the war years in exile in England and France, returning to Poland in 1951.

He worked as contributor to popular periodicals: Nowa Kultura (1950–1962), Szpilki (1953–73) and Przegląd Kulturalny.

In 1964 he was one of the signatories and the main author of the so-called Letter of 34 to Prime Minister Józef Cyrankiewicz regarding freedom of culture.