A founding member of the American Civil Liberties Union, she was involved in the campaign against the conviction in the Sacco-Vanzetti case. In 1936 Flynn joined the Communist Party and wrote a bi-weekly column for women’s rights for the Daily Worker, and chaired the women’s commission. In two years she was elected to the CP national committee. In 1942 Flynn ran for Congress in New York and received 50,000 votes.