[6] In 1862, his father rented the former house and estate of the late Dr. Floyd T. Ferris in uptown Manhattan as a summer residence.

[8] Isham, who was fluent in German and French, was, foremost, a historian with a reputation "as a learned scholar and brilliant intellectual.

[9] On September 2, 1891, Isham was married to Mary Todd "Mamie" Lincoln (1869–1938)[10] at the Brompton Parish Church in London, England.

[14][15] Isham and his wife bought a place in Manchester, Vermont, later known as the 1811 house, until they moved to New York City, residing at 19 East 72nd Street.

[16] Together, Charles and Mamie were the parents of one child:[10] After a short illness, Isham died at his residence, 122 East 38th Street in New York City, on June 9, 1919.