Widow of the 2d Marquis of Salisbury (James B. W. Gascoyne Cecil), she married the Earl of Derby in 1870.
Sources: Burke's peerage
(See the bibliography for full references to sources)
Mary Catherine Stanley (1824-1900), countess of Derby (born Sackville-West, other married name Gascoyne-Cecil, marchioness of Salisbury) was the sixth of the nine children of George Sackville-West, fifth Earl De La Warr. Due to her father's friendship with the duchess of Kent, Mary and her sisters were part of Princess (later Queen) Victoria's social circle during their youth. Her family had strong diplomatic, court, and military connections, and she showed early fascination with politics and diplomacy. In 1847 she married James Gascoyne-Cecil, second marquess of Salisbury, a contemporary of her father. His older age and status gave her an unusual degree of freedom, and she put her efforts into making Hatfield, their seat, a focus of rising Tory politicians, including Lord Carnarvon, Disraeli, and Hugh Cairns.
Although she did not entertain in London, she maintained Hatfield as a place for serious conversation between like-minded men. After James Gascoyne-Cecil's death she married Edward Henry Stanley, fifteenth earl of Derby. Without Hatfield, which she had lost on her first husband's death, she was not able to entertain as she could before, but she nonetheless formed a political circle around herself. She defended her husband's right to succeed Disraeli as Tory leader in 1876. From 1876 to 1880, she entered political discussion in her own right, influencing cabinet members and discussing government policy with the Russian ambassador. A series of rifts, however, between her and her former allies and her husband's expulsion from office caused her to withdraw from politics. Lady Derby was friendly with Darwin and corresponded with him on a number of subjects, from fish to spiritualism.
Relevant Gender Resources:
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-9208
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-8980
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-9209
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-8369
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-10157
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-10318
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-11146
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-11528
Darwin Correspondence Database, http://www.darwinproject.ac.uk/entry-13406
Ghosh, Peter. "Stanley, nee Sackville-West, Mary Catherine (1824 - 1900)", Oxford Dictionary of National Biography, Oxford University Press, 2004 (accessed March 5th, 2013).