Clark was married twice.
His first wife was Katherine Louise "Kate" Stauffer (1844 Pennsylvania – 1893 New York).

Together, they had seven children:
Mary Joaquina Clark (1870 – 1939), m. Everett Mallory Culver, Charles Potter Kling, and Marius de Brabant
Charles Walker Clark (1871 – 1933), m. Katharine Quin Roberts and Cecelia "Celia" Tobin
Unnamed son (1874–1874)
Jessie Clark (1875–1878)
Katherine Louise Clark (1875 – c. 1933), m. Dr. Lewis Rutherford Morris (1862-1936)
William Andrews Clark Jr. (1877–1934), m. Mabel Foster and Alice McManus
Paul Francis Clark (1880–1896)
After Kate's death in 1893, William married his second and final wife, the woman who had been his teenage ward, Anna Eugenia La Chapelle (March 10, 1878 Michigan – October 11, 1963 New York).
They claimed to have been married in 1901 in France.
Anna was 23 and William was 62.
They had two children:
Louise Amelia Andrée Clark (August 13, 1902 San Lucas, Spain – August 6, 1919 Rangeley, Maine)
Huguette Marcelle Clark (June 9, 1906 Paris, France – May 24, 2011, New York City)
In early 1946 Anna commissioned the Paganini Quartet, and acquired the four famous Stradivarius instruments once owned by Niccolo Paganini for their use.

William Clark donated 135 acres to the Girl Scouts in honor of his daughter, Louise Amelia Andrée, who died aged 16 of meningitis and had been very happy there.
The Girl Scout camp in Briarcliff Manor was named Camp Andree Clark.
Clark was married twice. His first wife was Katherine Louise "Kate" Stauffer (1844 Pennsylvania – 1893 New York).
Together, they had seven children:
Mary Joaquina Clark (1870 – 1939), m. Everett Mallory Culver, Charles Potter Kling, and Marius de Brabant
Charles Walker Clark (1871 – 1933), m. Katharine Quin Roberts and Cecelia "Celia" Tobin
Unnamed son (1874–1874)
Jessie Clark (1875–1878)
Katherine Louise Clark (1875 – c. 1933), m. Dr. Lewis Rutherford Morris (1862-1936)
William Andrews Clark Jr. (1877–1934), m. Mabel Foster and Alice McManus
Paul Francis Clark (1880–1896)
After Kate's death in 1893, William married his second and final wife, the woman who had been his teenage ward, Anna Eugenia La Chapelle (March 10, 1878 Michigan – October 11, 1963 New York). They claimed to have been married in 1901 in France. Anna was 23 and William was 62. They had two children:
Louise Amelia Andrée Clark (August 13, 1902 San Lucas, Spain – August 6, 1919 Rangeley, Maine)
Huguette Marcelle Clark (June 9, 1906 Paris, France – May 24, 2011, New York City)
In early 1946 Anna commissioned the Paganini Quartet, and acquired the four famous Stradivarius instruments once owned by Niccolo Paganini for their use.
William Clark donated 135 acres to the Girl Scouts in honor of his daughter, Louise Amelia Andrée, who died aged 16 of meningitis and had been very happy there. The Girl Scout camp in Briarcliff Manor was named Camp Andree Clark.
William A. Clark