Keen as he was to join the British forces fighting the Boers in 1899, the Secretary of State for War appointed him to chair a committee reviewing the future of the Yeomanry Force. He was then granted the honorary rank of Colonel and appointed Assistant Adjutant General for the Imperial Yeomanry, but the fighting was mostly over when he sailed for the Cape in 1901 with the temporary rank of Colonel in the Army
Lord Harris was appointed Honorary Colonel of the Royal East Kent Mounted Rifles in 1908.
When war broke out in 1914 Lord Harris was made a Vice-Lieutenant of Kent, responsible for organising the Kent contingent of the Volunteer Training Corps. The VTC rapidly expanded; by March 1915 there were 48 units and thousands of men to administer.
Controversially, Harris decided that Kent County Cricket Club should continue with its programme of matches after the outbreak of war, though this did not continue into 1915.