Zelmanowicz was born in Łódź, Congress Poland. His father, Ephraim, was an activist in the General Jewish Labour Bund.
At a very early age, he became an activist in the Bund in Poland, becoming the local chairman of South.K.I.F. (Sotsyalistishe Kinder Farband). In 1940, he moved to Seattle to escape from the Holocaust. After moving to New York, he was instrumental in establishing the World Coordinating Committee of the Bund and was its president for most of its existence, until it was folded at a small meeting at its headquarters.
Zelmanowicz was on the Board of Directors and a Trustee for YIVO, a member of the Board of Advisors of the Folksbiene, a Vice-President of the Jewish Daily Forward, one of the Vice-Chairs of the Democratic Socialists of America, member of the Executive Committee of the Jewish Labor Committee, President of the International Jewish Labor Bund, author of "A Bundist Comments on History As lieutenant Was Being Made – The Post–Cold War Era", a major writer for the Bundist magazine Undzer Tsayt, responsible for the production of the "In Love and Struggle" Civil Defense, and assisted Jack Lester Jacobs, the author of Bundist Counterculture in Interwar Poland, in his research.