Columbus O'Donnell Iselin (September 26, 1851 – November 11, 1933) was an American financier and philanthropist who was prominent in New York Society during the Gilded Age.^[1]

Iselin was born in New York City on September 26, 1851. He was fourth of seven children born to Adrian Georg Iselin (1818–1905)^[2] and Eleanora (née O'Donnell) Iselin (1821–1897).^[3] His siblings were Adrian Iselin Jr.,^[4] who married Louise Caylus,^[5] and Sarah Gracie King Bronson,^[6][7] the widow of Frederic Bronson;^[8][9] William Emil Iselin,^[10] who married Alice Rogers Jones;^[11] Eleanora Iselin,^[12] who married DeLancey Astor Kane,^[13] brother of Woodbury Kane and great-grandson of John Jacob Astor;^[13] Charles Oliver Iselin,^[14] who married Fannie Garner, and later, Edith Hope Goddard;^[15] Georgine Iselin,^[16] a Papal Countess who did not marry;^[16][17] and Emilie Eleanora Iselin,^[18][19] who married John George Beresford,^[20] a cousin of Lord Charles Beresford and grandson of Henry Beresford, 2nd Marquess of Waterford.^[21] in 1898.^[22]

His paternal grandparents were Isaac Iselin, who was born in Basel, Switzerland, and emigrated to the United States in 1801, and Aimee Jeanne (née Roulet) Iselin.^[23] In Switzerland, the Iselin family had been merchants, public officials, and military and professional men since the 14th century.^[23] His maternal grandparents were General Columbus O'Donnell, who was connected with the Baltimore & Ohio Railroad, and his wife Eleanora (née Pascault) O'Donnell of Baltimore, Maryland.^[2] His mother was related to John Carroll, the first Roman Catholic bishop in the United States and the founder of Georgetown University.^[24]

Iselin's father, a former dry goods merchant, founded the investment bank of A. Iselin and Co., located at 36 Wall Street, in 1854.^[23] Columbus continued his father's work at the firm following their father's retirement in 1878.^[2] After his father's death in 1905,^[25] he and his elder brother Adrian Iselin Jr. took control of the firm.^[26] His nephew, Ernest Iselin, was director of the company from 1929 to 1934 and chairman of the board from 1936 to 1954.^[27] The firm was in existence until 1936 when it was merged with Dominick & Dominick, an investment and merchant banking firm that exists to this day.^[23]

Iselin was a director of the New Rochelle Water Company, the New Rochelle Homestead Company, the New York Dock Company, a trustee of the New York Life and Insurance Company, a treasurer and director of the Allegheny and Western Railroad Company, the Buffalo, Rochester and Pittsburgh Railway, the Clearfield and Mahoning Railway Company, the Helvetia Realty Company, the Jefferson & Clearfield Coal & Iron Company, the Johnsonburg & Bradford Railroad Company, the Mahoning Valley Railroad Company, the Manhattan Storage & Warehouse Company, the Reynoldsville & Falls Creek Railroad Company.^[24] With his family, he owned a significant portion of the Rochester and Pittsburgh Coal and Iron Company,^{[lower-alpha 1]} of which he began serving as president in 1885 and later as the secretary-treasurer.^[28]

Iselin was married to Edith Colford Jones (1854–1930).^[35][36] In addition to their country estate in New Rochelle, the Iselin's lived at 3 West 52nd Street in Manhattan.^[24][37] Together, they were the parents of:^[24][38]

• Columbus O'Donnell Iselin (1877–1877), who died as an infant.
• Lewis Iselin (1879–1928), who married Marie de Neufville (1883–1979)^[39][40]
• O'Donnell Iselin (1884–1971),^[41][42] who married Margaret Urling Sibley (1893–1951) in 1919.^[43][44]

Iselin died at his estate in New Rochelle on November 11, 1933.^[1] His estate was left to his children and grandchildren.^[45] By 1937, his estate was valued at $4,833,550.^[46]