In the
January 1910 general election Leslie stood as the
Irish Parliamentary Party candidate for the
Londonderry City division, losing by just 57 votes. In the second general election later that year he was again narrowly defeated by the Unionist candidate. Before
World War I, Leslie travelled extensively and in 1912 he married Marjorie Ide, the youngest daughter of
Henry Clay Ide, then United States ambassador to Spain and former Governor-General of the
Philippines. His parents and other family members moved temporarily to London at the outbreak of war. During the war he was in a British Ambulance Corps, until invalided out; he was then sent to Washington, D.C. to help the British Ambassador, Sir
Cecil Spring Rice, soften Irish-American hostility towards England and obtain American intervention in the war in the aftermath of the 1916
Easter Rising in Dublin and the execution of its leaders. But he also looked to Ireland for inspiration when writing and edited a literary magazine that contained much Irish verse. He became a supporter of the ideals of
Irish nationalism, although not physical force republicanism. In the
1918 election the
Irish Parliamentary Party lost massively to
Sinn Féin, putting an end to Shane Leslie's political career, but as the first cousin of
Winston Churchill he remained a primary witness to much that was said and done outside the official record during the negotiation of the
Anglo-Irish Treaty of 1921. Disappointed, he felt unwanted in Ireland and abandoned by the British. Like many members of the landed gentry from the 1880s who were obliged to turn to other occupations, he could no longer rely on income from landholdings. He wrote extensively, in a wide range of styles, in verse, prose, and polemic, over several decades. His writings include
The End of a Chapter (1916), while hospitalised during the
Great War,
The Oppidan (1922), a
roman à clef about his life and contemporaries at Eton, an edition of the
Letters of Herbert Cardinal Vaughan to Lady Herbert of Lea (1942), and a biography
Mrs Fitzherbert: a life chiefly from unpublished sources (1939), together with an edition of her letters (with Maria Anne Fitzherbert),
The letters of Mrs Fitzherbert and connected papers; being the second volume of the life of Mrs. Fitzherbert (1944). He also wrote
Mark Sykes: His Life and Letters (1923), a biography of the English traveller, Conservative Party politician and diplomatic advisor. He advised budding novelist Scott Fitzgerald on the title of his 1st novel, they shared correspondence with the future Mnsg William A Hemmick who was Fitzgerald's teacher at the now shut Newman School. A passionate advocate of reforestation, he found the business of running an estate uncreative and boring, and transferred the estate entailed to him to his eldest son, John Norman Leslie, who succeeded as the 4th Baronet. He transferred
St Patrick's Purgatory on
Lough Derg to the Roman Catholic
Bishop of Clogher,
Eugene O'Callaghan. The wealth of the Leslies had waned by the 1930s following the
Wall Street crash of 1929 and a farm that was loss making. In his unpublished memoirs, he wrote "a gentleman's standing in his world was signalled by his list of clubs and it was worth paying hundreds of pounds in subs". They continued to maintain their lifestyle, involving attendance at the London season and the entertainment of distinguished visitors, including
Anthony Eden at Glaslough. At the outbreak of
World War II in 1939 he joined the
Home Guard. He spent the remainder of his life between Glaslough and London. ==Family==