Mills was the second son of Charles Mills, 2nd Baron Hillingdon, and the Honourable Alice Marion Harbord, daughter of Charles Harbord, 5th Baron Suffield.
[1] He succeeded his elder brother Charles Thomas Mills as Member of Parliament for Uxbridge in 1915, a seat he held until 1918.
Lady Hillingdon was chairman of the Central Women's Advisory Committee of the Conservative Party and was made a Dame Commander of the Order of the British Empire in 1939.
[4] She was godmother to her niece Sarah, eldest daughter of John Spencer-Churchill, 10th Duke of Marlborough.
[5] Lord Hillingdon died in December 1952, aged 61, and was succeeded in the barony by his eldest son, Charles.