St Clair Denny (1800-58), was the third son of Major Ebenezer Denny, who in 1816 became the first mayor of Pittsburgh. St Clair Denny graduated from West Point in 1822, and was in Washington at the same time as Whistler, serving as paymaster in the U.S. Army there from 1850 to 1856.
Denny and his wife Caroline (née Hamilton), had seven children. Anna Harding Denny, their third child, sat for her portrait to Whistler, Portrait of Anna Denny y004.
Young, Andrew McLaren, Margaret F. MacDonald, Robin Spencer, and Hamish Miles, The Paintings of James McNeill Whistler, New Haven and London, 1980 (cat. no. 4).
The Correspondence of James McNeill Whistler, 1855-1903, edited by Margaret F. MacDonald, Patricia de Montfort and Nigel Thorp; including The Correspondence of Anna McNeill Whistler, 1855-1880, edited by Georgia Toutziari. Online edition, University of Glasgow, 2004.