Born in Baltimore, Maryland, in 1925, Tanenbaum was the son of Orthodox Jewish immigrants from Russia. His interest in interfaith relations was sparked early by stories of Eastern European pogroms told around the dinner table by his parents. He studied at the Talmudical Academy and City High School of Baltimore before entering Yeshiva University in New York at the age of fifteen. After graduating with a degree in biology, he began rabbinical studies at the Jewish Theological Seminary (JTS). While a student at JTS, he started to study the roots of antisemitism among Christians and worked as an aide to Abraham Joshua Heschel, the renowned rabbi and professor of Jewish ethics and mysticism at the seminary. Tanenbaum subsequently edited several volumes of Heschel's books.
After his ordination as a Conservative rabbi in 1950, Tanenbaum briefly worked in publishing and public relations, and then went on to become one of the first Jewish professionals to focus primarily on Jewish-Christian relations. He served in the late 1950s as executive vice president of the Synagogue Council of America, a body of Orthodox, Conservative and Reform Jews that promoted fellowship and cooperation within the Jewish community. In 1960, he took a position with the American Jewish Committee (AJC) where he became director of interreligious affairs.
In the early 1960s, Tanenbaum's focus on interfaith work turned sharply to Jewish-Catholic relations when Pope John XXIII convened the Second Vatican Council. Tanenbaum saw this as an opportunity for overcoming centuries of church legitimated antisemitism and went to Rome as an official observer to the council. He enlisted his teacher, Abraham Joshua Heschel, as the chief Jewish spokesman in an effort to secure adoption of a council statement on Judaism and other non-Christian religions. Among the many changes to ultimately come out of Vatican II was the landmark document Nostra Aetate, which repudiated antisemitism and declared that the death of Jesus cannot be charged against all Jews, without distinction, then alive, nor against the Jews of today.
After Vatican II, Jewish-Catholic relations became Tanenbaum's primary concern, but he also worked to open avenues of understanding with Protestants (mainline as well as evangelical), Orthodox Christians and Muslims. In 1983, he became director of international affairs at the AJC and concentrated on such issues as international refugee relief, Middle East peace, and human rights, even as he continued his involvement with interfaith issues.
Tanenbaum was a member of White House commissions on children, the elderly and the Holocaust. He was a member of boards of directors of numerous institutions including the American Jewish World Service and the International Rescue Committee. As a pioneer in race relations, he served as a founder and program chairman of the National Conference on Race and Religion. He was founder and chairman of the National Interreligious Task Force on Soviet Jewry and served as chair of the International Jewish Committee for Interreligious Consultations. He worked as a consultant to movie and television productions on religious and Jewish subjects.
Beginning in 1965, Tanenbaum hosted a syndicated broadcast on radio station WINS in New York City where his discussions on a range of subjects, mainly religious, reached a wide audience. He was also a popular lecturer, author, editor and critic and published numerous articles and books.
Among the many honors he received were fifteen honorary degrees from both religious and secular institutions,as well as, the International Interfaith Achievement Award of the National Conference of Christians and Jews, and the Israel and Libby Mowshowitz Award of the New York Board of Rabbis. Presidents Kennedy, Carter, Reagan and Bush sought his advice and counsel.
Tanenbaum retired from the American Jewish Committee in 1988, but remained on as a consultant. He was married to Helga Weiss Tanenbaum with whom he had three children, Susan, Michael and Adena. The marriage ended in divorce, and in 1982, he married Dr. Georgette Bennett. They had one son, Joshua Marc. Rabbi Marc H. Tanenbaum died on 3 July 1992.