 Ida Farrer (1854–1946), was the daughter of Thomas Farrer, 1st Baron Farrer.Her younger sister Nora, later became Lady Barlow after marrying Sir Alan Barlow, 2nd Baronet, while her elder brother Erasmus was killed during the Second Battle of Ypres in 1915

In 1948, she married the Welsh psychiatrist William Rees-Thomas, who was a colleague of hers on the Board of Control.She died in 1972, predeceasing her husband.