During this period of her life Flexner found her way into New York's radical left. She joined the Communist Party in 1936 and spent several years writing CP articles and pamphlets, under pseudonyms, and working for various social and political causes. As a member of the League of American Writers, she served on its "Keep America Out of War Committee" in January 1940 during the period of the Hitler-Stalin pact. She worked alongside the National Association of Colored Graduate Nurses. In 1946, she became the executive director of the Congress of American Women This activist background allowed Flexner to appreciate the disappointments, triumphs, and bracing camaraderie experienced by the 19th- and early 20th-century women whom she later described in "Century of Struggle".