Coffin was born February 13, 1922 in San Marino, California, the son of Tristram Roberts Coffin, an investment banker formerly of Richmond, Indiana and New York City, and Elsie Potter Robinson of Edgewood Farm, Wakefield, Rhode Island.
Coming to Rhode Island after his father died of influenza in 1927, he was educated at the Providence Country Day School, Moses Brown School (1939) in Providence, and then Haverford College (1943) outside of Philadelphia.