Braithwaite studied at the Royal Military College, Sandhurst, and was commissioned as a lieutenant in the Somerset Light Infantry on 3 January 1886. He was promoted to captain on 8 November 1894. He served in the Second Boer War, seeing action at Ladysmith, Spion Kop, Vaal Krantz and Tugela Heights. He was mentioned in despatches three times and in a South Africa honours list received the brevet rank of major on 29 November 1900. Staying in South Africa until the war ended, he only returned to the United Kingdom on the SS "Briton" three months later in September 1902. After his return he was on in early October posted to Southern Command as a deputy assistant quartermaster-general on the staff of Sir Evelyn Wood, General Officer in Command of the 2nd Army Corps. In 1906, Braithwaite was promoted to major, and transferred to The Loyal North Lancashire Regiment. He was later promoted to lieutenant colonel, and served as an instructor at the Staff College, Camberley. In 1909, he was assigned to the staff of Douglas Haig at the War Office, and promoted to colonel. He was subsequently named commandant of the Staff College, Quetta, a position he still held at the outbreak of the First World War. At this point, the college was closed, and he was again transferred to the War Office, this time as Director of Staff Duties.