St. Leger and his unit took part in the cavalry charge at Klip Drift, in the relief of Kimberley, the occupation of Bloemfontein, the battles of Sand River and Doornkop and the march on Pretoria in June 1900.

During his military campaigns he carried sketch-books in his haversack at all times; he made rough pencil sketches when the opportunity was available and would later work them into final drawings or water colours.

[4] In addition to being a detailed account of the experiences of the 1st Mounted it contained full-page reproductions of sixty-six of his sketches in water colour and pen and ink, plus ninety-seven line drawings and five section maps.

St. Leger mounted an exhibition of his work at a gallery in Bruton Street, Mayfair in May and June 1904 and consisted of fifty water colours with fifteen large and a great many smaller pen and ink drawings.

At the outbreak of the First World War, St. Leger rejoined the Royal Irish and as part of the British Expeditionary Force crossed the English Channel and proceeded to the front.