Margaretta Drexel was the granddaughter of Anthony Joseph Drexel (1826-1893), the Philadelphia financier and philanthropist, partner of J.P. Morgan, and founder of Drexel University. His niece, Saint Katherine Drexel, was sainted in 2000, only the second American in history. In 1910, Margaretta married Guy Finch Hatton, 14th Earl of Winchilsea and Nottingham. Her brother-in-law was Denys Finch Hatton, the African big game hunter and adventurer featured in Isak Dinesen's (Karen Blizen's pseudonym) Out of Africa.
(Sothebys)
The work was undated by Sothebys but I'm guessing it was done shortly after her marriage. The hair looks about right.
Sold Sothebys, New York, Wed, 24 May 06, Sale # N08207, LOT 56 (of 246)); Hammer Price with Buyer's Premium: 168,000 USD; estimated 40,000—60,000 USD