Her father died before inheriting the Earldom and it passed to her brother, William Wentworth-FitzWilliam, 7th Earl FitzWilliam[2] whose lifestyle she criticised; "he had so much and everyone else had so little".
[3] She married Lt. Col. William Mackenzie Smith on 29 July 1899 and went to live in Barnes Hall near Grenoside, Sheffield.
[11] She stated that her social conscience developed after seeing the conditions of children who lived on the Wentworth estate.
[4] Lady Mabel's niece, Joyce Smith, described her as a "rabid socialist" whose name was "absolutely taboo" at Wentworth Woodhouse.
The college later merged with Sheffield City Polytechnic and the name Wentworth Woodhouse was restored.