Following his Charterhouse education, Maitland attended the Royal Military College Sandhurst before he was commissioned into the Royal Scots Fusiliers as a second lieutenant on 6 December 1899. He served with the regiment in South Africa during the Second Boer War, where he was wounded. While there, he was promoted to lieutenant on 12 November 1900, and for his service he received the Queen's South Africa Medal with four clasps. Having returned to the United Kingdom before the end of war in June 1902, he was again sent to South Africa to serve there with the 2nd battalion in November 1902, then served in the United Kingdom until 1906, when he was transferred to the first battalion in India. After leaving there in 1910, he trained at the Staff College, graduated three years later and then served in the First World War. He was promoted to brigade major in April 1915, and to be brevet major that June. He suffered severe injuries at Ypres, was mentioned in despatches eight times, received the Distinguished Service Order in 1916, and was appointed a Companion of the Order of St Michael and St George in 1919.