Winsor, a freelance writer in Boston, was appointed a trustee of that city's public library (1866) and then became its chief administrator, at first on a temporary basis. During his tenure of office he established numerous branch libraries in Boston. In 1876 he was a founder of the American Library Association and became its first president, serving until 1885 and again in 1897. Winsor also was a historian; he edited the Narrative and Critical History of America, 8 vol. (1884–89), and wrote several books.