Following the outbreak of the Second Boer War he volunteered for service with the Imperial Yeomanry, and served in South Africa from 1899 to 1902.
In October 1900 he was commissioned a second lieutenant in the 1st battalion of the King's Own Scottish Borderers, stationed in South Africa, and the following year served in Transvaal, west of Pretoria.
[1] He later served in World War I, having rejoined the army in the Reserve of Officers, and in 1918 was a captain in the Bedfordshire Regiment.
In 1920, he inherited the title from his brother to become 18th Lord St John, and lived at Melchbourne Park.
[citation needed] St John was a member of the Electoral Committee of the Houses of Parliament from 1925 to 1931.