Onslow joined the army on the outbreak of World War I in 1914, being commissioned as a second lieutenant on 15 June 1915.

[1] He was mentioned in despatches three times, received an OBE and the French Legion of Honour.

[6] Onslow devoted much of his retirement to writing, producing The Empress Maud (1939); Sixty-three Years: Diplomacy, the Great War and Politics, with Notes on Travel, Sport and Other Things (1939), which went through several editions; and The Dukes of Normandy and Their Origin (1945), which was completed in the year of his death and published posthumously.

They had two children: Lord Onslow died on 9 June 1945, aged 68, and was succeeded in the peerage by his only son.

As Dowager Countess of Onslow, Violet gave the future Queen Elizabeth II a diamond and ruby butterfly brooch as a wedding gift in 1947.