In the late 1950s and early 1960s, Gendler served as rabbi to a number of congregations throughout Latin America, including the Beth Israel Community Center in Mexico City, Mexico (1957–1959), the Associacao Religiosa Israelita in Rio de Janeiro, Brazil (1961), and the five congregations of Havana, Cuba (High Holidays and Passover, 1968–1969).
During the late 1960s and early 1970s, Gendler, along with his wife Mary Gendler (born 1940) was involved in several alternative residential communities, including Ivan Illich's Centro Intercultural de Documentación in Cuernavaca, Mexico (1968–1969) (alongside Harvey Cox[5]) and the inter-racial inter-religious living center Packard Manse in Stoughton, Massachusetts (1969–1971).
During the 1960s, he played a pivotal role in involving American Jews in the movement, leading groups of American Rabbis to participate in prayer vigils and protests in Albany, Georgia (1962), Birmingham, Alabama (1963) and Selma, Alabama (1965), and persuading Abraham Joshua Heschel to participate in the famous march from Selma to Montgomery (1965).
In 2021, his translation of the works of Rabbi Aaron Samuel Tamares was published by Ben Yehuda Press, to critical acclaim.
During the 1960s and 1970s, inspired in part by the work and writings of his friends and mentors Helen and Scott Nearing, Gendler became involved in the conservation and environmental movements, and was an advocate and practitioner of organic farming and vegetarianism.
In 2013, he was honored with the Presidents' Medallion from Hebrew Union College-Jewish Institute of Religion in recognition of "a life of passionate devotion to humankind" and a "lifetime of commitment to social justice and environmentalism."