After the German invasion of Poland in 1939, Julian Grobelny moved to Warsaw, where he supported the banned Polish Socialist Party (PPS) under the code name of Trojan. Grobelny witnessed the persecution of Warsaw’s Jews and decided to help them. In 1943 he took over as head of the underground organization Żegota. In this function, he expanded Żegota’s activities and raised its budget. He also took action against informers. He and his wife Halina (1900–1993) hid Jews in their house near Warsaw. Jewish children in particular found refuge there before being placed with Catholic families. During the 1943 uprising in the Warsaw ghetto, Grobelny supported Jewish fighters. He was arrested by the Gestapo in March 1944. A month later, his friends from the underground movement managed to smuggle him out of a prison hospital. He then hid in Otwock, but continued his resistance work. He died of tuberculosis in December 1944.
Grobelny was honored as Righteous Among the Nations in 1987.