Born in Newton Abbot, Devon, to Captain Joseph Sparkhall Rundle, a Royal Navy officer, and his wife Renira Cathrine (née Leslie, who was the daughter of Commander W. W. Leslie of the Royal Navy), Leslie Rundle was commissioned into the Royal Artillery in 1876. He fought in the Zulu War in 1879, the First Boer War of 1881 and the Anglo-Egyptian War of 1882. He was involved in the Nile expedition between 1884 and 1885 and served in the Sudan Frontier Field Force from 1885 to 1887. For service in the Khartoum expedition of 1898 he was promoted to Major-General for distinguished conduct in the field. He led a column up the Blue Nile to relieve Gedaref the same year.