Born in Cleveland to Elizabeth (Gwinn) and Samuel Livingston Mather, he attended Cheshire Academy in Connecticut, and received his A.B. in 1877 from Trinity College.
Mather entered the family business, the Cleveland Iron Mining Co., as a clerk and worked his way up to vice-president in 1885. He succeeded his father as president in 1890. In 1891 the business merged with the Iron Cliffs Co. to form the Cleveland-Cliffs Iron Co. and Mather remained president until 1933. Under Mather's command the company grew and diversified into ore-related industries by acquiring coal mines in Pennsylvania and West Virginia. In 1933 Mather became Chairman of the Board and, in 1947, he was named Honorary Chairman.
Mather was the first Chairman of the Chamber of Commerce-City Planning Committee, 1899-1911. In 1930 he helped form the REPUBLIC STEEL CORP. and served as a director. Mather was also Director of the Kelley Island Lime & Transport Co., and Chairman of the Lake Superior & Ishpeming Railroad Co.
In 1933 the Cleveland Chamber of Commerce awarded Mather the medal for distinguished public service. He was a trustee of many institutions including the Cleveland Museum of Art, UNIVERSITY HOSPITALS OF CLEVELAND, Western Reserve University (see CASE WESTERN RESERVE UNIVERSITY), and the WESTERN RESERVE HISTORICAL SOCIETY.
Mather married Elizabeth Ring Ireland (see ELIZABETH RING IRELAND MATHER) on 18 May 1929. He had no children of his own and was survived by his wife and her son from a previous marriage, James Duane Ireland. Mather is buried in LAKE VIEW CEMETERY.
The William Gwinn Mather Family Papers, WRHS.