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 October 1969, at his father's request, the thirteen-year-old Ellsberg and his sister helped photocopy pages of the Pentagon Papers, in an effort to teach them "the importance of following one's conscience".

[3] In 1975, at age 19, Ellsberg dropped out of Harvard, intending to spend a few months with the Catholic Worker Movement in New York City.

[4] In 1978 Ellsberg and his father were arrested outside the Rocky Flats Plant, a manufacturing complex, while protesting nuclear armaments.

He has written a number of articles for the Jesuit Review, America,[7] the National Catholic Reporter,[8] and Maryknoll Magazine.