He married Mary Ann Grigsby (1828–1878) on July 11, 1848, daughter of Virginia planter Reuben Grigsby of
Hickory Hill plantation. Together, William and Mary were the parents of five children: • Emma Louise McCormick (1854–1893), who married Wisconsin and Chicago judge, politician and railroad executive
Perry H. Smith on June 18, 1878. • Anna Reubenia "Ruby" McCormick (1860–1917), who married
Edward Tyler Blair (1857–1939) on May 29, 1882. • Lucy Virginia McCormick (1865–1928), who married lawyer
Samuel Rountree Jewett (1862–1922), on October 9, 1888. Outwardly, McCormick was described as "of a cheerful disposition, quick spoken, ... a most interesting and agreeable man". He was buried in
Graceland Cemetery, and replaced as general manager by
Charles A. Spring Jr. Descendants Through his son Robert, he was the grandfather of
Joseph Medill McCormick (1877–1925), a
U.S. Senator from Illinois, and
Robert R. McCormick (1880–1955), the publisher of the
Tribune, which had been a major attacker of the family during the war. and the great-grandfather of
Brooks McCormick (1917–2006), who was the last McCormick to lead the family firm. Through his daughter Ruby, he was the grandfather of financier
William McCormick Blair (1884–1982) and great-grandfather of
William McCormick Blair Jr. (1916–2015), who served as
U.S. Ambassador to Denmark from 1961 to 1964 and as
the Philippines from 1964 until 1967. ==Family tree==