His family immigrated to New York in 1906. Gellert studied in art schools in New York. His illustrations were first published in radical Hungarian and American magazines, but in the 1920s Gellert worked as a staff artist for "The New Yorker" magazine and "The New York Times" newspaper. Although he was opposed to United States' entry into World War I, when conditions were worsening in Europe in 1939 after the rise of Nazi Germany, Gellert helped organize "Artists for Defense"; he later became chairman of "Artists for Victory", which included over 10,000 members.