William J. Mayo married Hattie Marie Damon (1864-1952) in 1888. They had 3 children—William Damon Mayo, Worrall Mayo, and Helen Phoebe Mayo—between 1889 and 1893. Sadly, all of them died during infancy. "Dr. Will" himself died in July 1939 of gastric carcinoma (stomach cancer) in Rochester, Minnesota. Ironically, that tumor had been a major focus of his surgical practice. Mayo is buried with his family members near his parents and brother at Oakwood Cemetery in Rochester.

On August 21, 1883, a terrible tornado struck Rochester, killing 29 people and seriously injuring over 55 others. One-third of the town was destroyed, but young Will and his family escaped serious harm. The relief efforts began immediately with a temporary hospital being established at the town's dance hall. The Mayo brothers were extensively involved in treating the injured who were brought there for help Mother Alfred Moes and the Sisters of Saint Francis were called in to act as Nurses (despite the fact they were trained as teachers and had little if any medical experience)

After the crisis had subsided, Mother Alfred Moes approached william Worrall Mayo about establishing a hospital in Rochester. On September 30, 1889, the dream became reality as Saint Mary's Hospital opened its doors. W.W. Mayo, 70 years old, became the consulting physician and surgeon at the hospital, and his two sons began seeing patients and performing surgery with the assistance of the Sisters of Saint Francis.

Augustus Stinchfield was also asked to join the medical practice in 1892 by william Worrall Mayo. Once Stinchfield was hired, W. W. Mayo retired at age 73. Others who were invited to be part of the enterprise were C. Graham, E. Starr Judd, Henry Stanley Plummer, Melvin Millet and Donald Balfour. This group of doctors, along with their dedicated staff, created the world's first private integrated group practice. The generosity of the Mayo brothers and their partners, together with their shared industry, ensured the Future growth of the Mayo Clinic.

William J. Mayo served in World War I at the rank of Colonel (O6), as chief adviser for U.S. Army surgical services in the office of the U.S. Army Surgeon General. His brother Charles was also a Colonel in the U.S. Army Medical Corps and alternated with william as the associate chief adviser for surgical care of U.S. Army Soldiers. When President Woodrow Wilson organized the Committee of American Physicians for Medical Preparedness in 1916, william was named its chairman and Charles as one of its members. That committee subsequently became the General Medical Board of the Council for National Defense. william was made a member of its executive committee and Charles was his alternate.

After hostilities ended in November 1918, both of the Mayo brothers were promoted to the rank of Brigadier General (O7) in the U.S. Army Reserve. In addition, they both received the U.S. Distinguished Service Medal for their wartime Service to the country.

In September 1931, Mayo and other prominent individuals of the time were invited by The New York Times to make a prediction concerning the world in eighty years time in the Future, in 2011, to celebrate the paper's hundred-and-sixtieth anniversary since its establishment in 1851. Mayo's prediction was that the life expectancy of developed countries would reach 70 years, compared to less than sixty years in 1931.

The United States Postal Service printed a stamp depicting him and his brother, Charles Horace Mayo, on September 11, 1964.

"Three-score and ten" means 70 years. The average life expectancy in the United States was approximately 78 years as of 2010, and it exceeded 80 years in other countries such as Japan.