Robert is the son of Carol Cummings, the daughter of a Marine Corps Brigadier General, and the American military analyst and whistleblower, Daniel Ellsberg. In 1975, at age 19, he dropped out of college, intending to spend a few months with the Catholic Worker Movement.
He stayed to become the managing editor of The Catholic Worker for two years (1976-8), a job that would introduce him to Dorothy Day and consequently would allow him to work with Day for the last five years of her life. This life-changing experience prompted him to convert to Catholicism.
In 1987 he began work as editor-in-chief of Orbis Books. He is the author of several books, many of which have received awards; his Blessed Among All Women tied a Catholic Press Association record by winning awards for Gender, Spirituality, and Popular Presentation of the Catholic Faith. He is the editor of the published diaries and letters of Dorothy Day.