Following the outbreak of the Second Boer War in late 1899, he volunteered for service and was appointed a lieutenant in the Imperial Yeomanry on 24 February 1900,[2][3] leaving England for South Africa on the SS Manchester Merchant in early March.

At the start of World War I, he was appointed to the staff of the newly formed 10th (Irish) Division under Lieutenant-General Sir Bryan Mahon at the Curragh Camp.

At this time, the Lord Lieutenant was largely a ceremonial position; real power was in the hands of the Chief Secretary and the Under-Secretary.

[11] He insisted on being kept up to date on the state of the country, and had the Under-Secretary, Sir Matthew Nathan, send him police reports, details of prosecutions and recruitment figures.

[12] On the weekend preceding the Easter Rising, following the capture of the German arms ship Aud and the arrest of Sir Roger Casement, Wimborne urged Nathan to order the arrest of a large number of rebel leaders.

[14] In June 1916 he directed the establishment of the Property Losses (Ireland) Committee to assess the many insurance claims resulting from the Rising.

[18] They had one son, Ivor Guest, 2nd Viscount Wimborne, and two daughters, Rosemary and Cynthia.