Son of the Baron Benon de Tusch-Letz and Adela Safrin, he was born on 6 March 1909 in Lwów (then Lemberg, Austro-Hungarian Empire, now Lviv) to a Galician-Jewish nobilitated family. The family moved to Vienna at the onset of First World War, and Lec received his early education there. After the war the family returned to Lviv (then Lwów in the Second Polish Republic) to continue his schooling at the Lemberg Evangelical School. In 1927 he matriculated at Lwów's Jan Kazimierz University in Polish language and law.
His literary debut was in 1929. Much of his early work was lyric poetry appearing in left-wing and communist magazines. He collaborated with the communist “Dziennik Popularny" between 1933 and 1936. In 1935 he co-founded the satirical magazine Szpilki (Pins). A "literary cabaret" he founded in Lwow in collaboration with Leon Pasternak in 1936 was closed by the authorities after several performances. Nor did his law-abiding image improve after he took part in the Convention of Culture Workers, a radical congress initiated by the international communist movement Popular Front in the same year. Later that year he spent a few months in Romania, afraid that his activism could lead to his arrest in Poland. He spent the next two years in Warsaw, where he was involved with a number of other left-leaning publications.
Following the German invasion of Poland on 1 September 1939, Lec fled Warsaw, returning to his hometown, Lwów. Lec spent the years 1939–1941 there, while the city along with the rest of Polish Eastern Borderlands was occupied by the Soviet Union after the Soviet invasion of Poland on 17 September 1939. While in the Soviet Union, Lec joined in the literary life under the auspices of Ukrainian SSR authorities. He contributed to the magazine “New Horizons”. His poems, satires, articles, and translations from Russian were published in ”Krasnoe Znamya” magazine. In 1940 he joined the Union of Soviet Writers of Ukraine and became a member of the editorial board of “The Literary Almanac” in Lvov. Through this and similar activities he became one of the most prolific pro-Soviet Polish writers, producing numerous works praising the Soviet regime. including one of the first poems to glorify Stalin written in the Polish language. A number of his works appeared in the Czerwony Sztandar (Red Banner) magazine. On 19 November 1939 Lec signed a resolution calling for the incorporation of Polish Eastern Borderlands into the territory of the Soviet Union. Lec's collaboration with the Soviet authorities remains controversial to this day, through he has been defended by Adam Michnik who wrote in his 2007 book that Lec has been unfairly branded by critical opinion as a "Soviet collaborator" on the basis of his "weakest, least successful, or most frankly conformist pieces".
After Nazi Germany's attack on the Soviet Union he was imprisoned in a German work camp in Tarnopol (now Ternopil), from which he made several attempts to escape. He received a death sentence for his second attempt to escape, but managed to successfully escape in 1943 after killing his guard with a shovel when taken to dig his own grave. This became the subject of one of his most famous poems "He who had dug his own grave" (from the cycle "To Abel and Cain"):