John S Churchill, or "Jack" was born in Dublin on 4 February 1880, younger son of Lord and Lady Randolph Churchill. Jack was educated at Elstree School, Hertfordshire, and like his elder brother Winston, at Harrow. In August 1908 he married Lady Gwendeline Bertie, daughter of the 7th Lord Abingdon, having two sons, John G Churchill and Henry Churchill, or "Peregrine", and one daughter, Clarissa Churchill, later Lady Eden and then Lady Avon.

On leaving [? the University of Cambridge], Jack entered the employment of the merchant banker and financier Ernest Cassel, before becoming a lieutenant in the South African Light Horse during the Boer War, where he was wounded and also mentioned in despatches. During the First World War, he served in France as a major with the Oxfordshire Hussars, becoming first a staff officer with Field Marshal Sir John French [Commander-in-Chief of the Expeditionary Forces in France, later 1st Lord Ypres] from 1914-15, then joining the staff of General Sir Ian Hamilton [commander, Mediterranean Expeditionary Force] in 1915 at the Dardanelles, before returning to France on the Anzac staff of General Birdwood [General Officer Commanding Australian and New Zealand Army Corps] from 1916-18. After the war, Jack returned to a business career and became a stockbroker (despite having previously hated the idea of going into the City). He died on 23 February 1947.