Opposed to World War I, Gellert published his first anti-war art in 1916. His work was prominently featured both in the illustrated magazine of the Hungarian Socialist Federation of the Socialist Party of America, "Előre" (Forward), as well as Max Eastman's radical monthly magazine "The Masses" from this time. He also created numerous illustrations for Eastman's successor magazine, "The Liberator," as well as sundry publications of the Communist Party USA after its formation, such as "The Workers Monthly" and "The New Masses." Later, Gellert was offered a position as a staff artist for "The New Yorker" magazine. In 1925, he moved to the "New York Times".