Lionel Sadleir-Jackson was born on New Year's Eve 1876 to Major Henry Sadleir-Jackson of Midleton, County Cork. He joined the South Staffordshire Regiment of the Militia as a second lieutenant in 1896, and was promoted to lieutenant in 1897. He later took a regular army commission in the 9th Lancers, reverting to second lieutenant, aged 21 in May 1898. Shortly after receiving his regular commission he was dispatched to South Africa with his regiment to participate in the Second Boer War. During the war he was posted as a staff officer and saw extensive action, including at the battles of Belmont, Modder River and Magersfontein and Paardeberg, culminating in the relief of Kimberley. For his service in these actions, Sadleir-Jackson was promoted to lieutenant and continued service in the Orange Free State and the Transvaal.