In order to supply the Allied Forces need for steel during World War II, the Mather led a convoy of 13 freighters in early 1941 through the ice-choked Upper Great Lakes to Duluth, Minnesota, setting a record for the first arrival in a northern port. This heroic effort was featured in the April 28, 1941 issue of Life Magazine. She was one of the first commercial Great Lakes vessels to be equipped with radar in 1946. In 1964, she became the very first American vessel to have an automated boiler system, manufactured by Bailey Controls of Cleveland, Ohio.
In 1985, Cleveland-Cliffs sold its two remaining operating steamers to Rouge Steel Company, and gradually sold off its idle vessels until only the Mather remained, laid up in Toledo, Ohio where she had been since 1980. On December 10, 1987, Cleveland-Cliffs, Inc. donated the steamer William G. Mather to the Great Lakes Historical Society to be restored and preserved as a museum ship and floating maritime museum. After she was brought to Cleveland in October of 1988 and funding acquired from local foundations, corporations, and individuals, restoration began. Fire damage to the Mather's galley and after cabin spaces required a major restoration effort. All over the vessel, most of the work was supplied by volunteers who repaired, cleaned, chipped, painted, and polished brass in order to restore the Mather's former elegance. In October 1990, she was moved to her permanent berth at the East Ninth Street Pier on Cleveland's North Coast Harbor.
In Sept. 1994 the Great Lakes Historical Society divested itself of the museum. Due, in large part, to a groundswell of local support to keep the Mather in Cleveland, the Harbor Heritage Society was created to negotiate a new lease agreement with the city. Incorporated in June 1995, Harbor Heritage formally acquired the Mather on 22 July 1995, and in 1996 continued to oversee the Mather's ongoing restoration, promotion, and development as a historic vessel. After 10 years of negotiations, the City Of Cleveland, represented by Mayor Jane Campbell signed a 40 year lease on 15 June 2003 allowing the Mather to stay at it's East 9th berth.
On July 30, 1995 the Steamship William G Mather was dedicated as an American Society of Mechanical Engineers National Historic Mechanical Engineering Landmark for her 1954 installation of a single marine boiler and steam turbine engine, her 1964 installation of the Bailey 760 Boiler Control System and American Shipbuilding AmThrust dual propeller bow thruster - all firsts for U.S.-Flag Great Lakes vessels.