Original last name: Hirschhorn. Son of an innkeeper, Isaak Hirschhorn, who was also a gabbai in Tarnów, and Necha (née Schneider). Rudnicki spent his childhood and youth in Tarnów. He studied at a trade school in Warsaw, which he completed in 1931 and took up a job as a bank clerk.
He debuted in 1930 by publishing the novella Śmierć Operatora [Death of the Operator] in Kurier Poranny (issue 234). He cooperated with this journal until 1934, publishing, amongst other works, reports from the life of small-town Jews entitled Wizyta w Górze [Visit to the Up Above] (in 1933, issues: 292-301, 316-318). He also wrote short stories and novels, fragments of which were published in the magazines Na Szerokim Świecie, Gazeta Polska, Kurier Literacko-Naukowy, Wiadomości Literackie, Czas (in which, amongst others, the well-known Kartki Chasydzkie [Chasidic Cards] were published in 1934) and Miesięcznik Literatury i Sztuki. It is known from various accounts that he worked on the novel Dzieci [Children], the manuscript of which was burnt at the very start of the war in 1939.
Rudnicki was amongst the founders of the Przedmieście (Suburbs) literary group, which also included: Helena Boguszewska (chairman), Jerzy Kornacki, Władysław Kowalski, Gustaw Morcinek, Zofia Nałkowska, Bruno Schulz, Halina Krahelska and Józef Łobodowski. The programme of Przedmieście was formulated by Boguszewska and Kornacki in the ‘seven point declaration’. It postulated the use of new methods of observation, strongly associated with the naturalist tradition under the influence of Emil Zola and the French naturalists, interest in the lives of workers and national minorities, collective work of writers and scientists, and promoted factographic tendencies in literature. However, soon after the group was founded, Rudnicki and Krahelska left it. The group existed until 1937 and then disbanded voluntarily.
In 1933, Rudnicki became a member of the ZZLP (Związek Zawodowy Literatów Polskich – Trade Union of Polish Writers), which was established at the first all-district Congress of Polish Writers on 12-14th May 1920 in Warsaw. The organisation aimed to take care of writers’ interests: keeping copyrights, due royalties, other social securities and scholarships.
After the outbreak of World War II, he participated in the 1939 September Campaign and fought in Modlin. After the capitulation of Modlin, he was deported to East Prussia. In 1940, he managed to escape from captivity and get to Lviv, which was occupied by the Soviet army. There he cooperated with the magazines Nowe Widnokręgi and Czerwony Sztandar. In September 1940, he became a member of the Union of Soviet Writers of Ukraine.
After Lviv had been taken over by the German army, he got through to Warsaw, where he took an active part in the underground life, hiding under the name of Leonard Heryng, who was murdered in Auschwitz. He was a liaison officer of the support unit for writers and artists in the Jewish National Committee. He took part in the Warsaw Uprising in 1944 as a Home Army soldier. After the fall of the Uprising, he stayed in Milanówek near Warsaw, then he moved to Lublin, and at the beginning of 1945 – to Łódź.