On November 26, 1895, he married Edith Rockefeller (1872–1932), the youngest daughter of Standard Oil co-founder John D. Rockefeller and schoolteacher Laura Celestia "Cettie" Spelman.
McCormick became the third inaugural trustee of the Rockefeller Foundation.
He was also a trustee of the Rockefeller-created University of Chicago.
 He and Edith resided at 1000 Lake Shore Drive in Chicago and were the parents of five children before their divorce in December 1921:
John Rockefeller McCormick (1897–1901), who died young from Scarlet fever.

Editha McCormick (1903–1904), who also died young.

Harold Fowler McCormick Jr. (1898–1973), who married Anne Urquhart Brown "Fifi" (née Potter) Stillman (1879–1969), who had previously been married to James A. Stillman, and was the daughter of James Brown Potter and Mary Cora Urquhart.

Muriel McCormick (1903–1959), who married Elisha Dyer Hubbard (1878-1936), a nephew of Elisha Dyer Jr. and grandson of Elisha Dyer (both Rhode Island governors), in 1931.

Mathilde McCormick (1905–1947), who married Wilheim Max Oser (1877–1942), a Swiss riding instructor, in April 1923.

After his divorce from Edith, and before his second marriage, McCormick sought to fortify himself by undergoing an operation by Serge Voronoff, a surgeon who specialized in transplanting animal glands into aging men with impotency.
 In 1922, McCormick married Polish opera singer Ganna Walska.
They divorced in 1931.

McCormick died on October 16, 1941, of a cerebral hemorrhage, at his home in Beverly Hills, California.
On November 26, 1895, he married Edith Rockefeller (1872–1932), the youngest daughter of Standard Oil co-founder John D. Rockefeller and schoolteacher Laura Celestia "Cettie" Spelman. McCormick became the third inaugural trustee of the Rockefeller Foundation. He was also a trustee of the Rockefeller-created University of Chicago.  He and Edith resided at 1000 Lake Shore Drive in Chicago and were the parents of five children before their divorce in December 1921:
John Rockefeller McCormick (1897–1901), who died young from Scarlet fever.
Editha McCormick (1903–1904), who also died young.
Harold Fowler McCormick Jr. (1898–1973), who married Anne Urquhart Brown "Fifi" (née Potter) Stillman (1879–1969), who had previously been married to James A. Stillman, and was the daughter of James Brown Potter and Mary Cora Urquhart.
Muriel McCormick (1903–1959), who married Elisha Dyer Hubbard (1878-1936), a nephew of Elisha Dyer Jr. and grandson of Elisha Dyer (both Rhode Island governors), in 1931.
Mathilde McCormick (1905–1947), who married Wilheim Max Oser (1877–1942), a Swiss riding instructor, in April 1923.
After his divorce from Edith, and before his second marriage, McCormick sought to fortify himself by undergoing an operation by Serge Voronoff, a surgeon who specialized in transplanting animal glands into aging men with impotency.  In 1922, McCormick married Polish opera singer Ganna Walska. They divorced in 1931.
McCormick died on October 16, 1941, of a cerebral hemorrhage, at his home in Beverly Hills, California.
Harold Fowler McCormick