He married twice: 1. Eliza McNeill (d. 1855), by whom he had one child: William McNeill Flagg (b. 1852). 2. Mary L. Ferris.
He studied civil engineering, and was educated at Hopkin Grammar School, New Haven, CT; Trinity College, Hartford, CT; New York University and St John's College, Annapolis, MD. He became a Deacon in 1848, and priest in 1849. He held several pastorates and founded four churches. He was Rector of All Saints' Church, NY; Church of the Resurrection, NY, and assistant of Grace Church, NY. He was secretary of the Society of American Authors. His publications included Earlier and Later Poems (1907).
N. G. Flagg and L. C. S. Flagg, Family Records of the Descendants of Gershom Flagg of Lancaster, MA, 1907, p. 127; Who Was Who in America, vol. 1, 1897-1942, Chicago, 1943.