William Livingstone Hatchwell Sinclair was born at Kirkmanshulme near Manchester, Lancashire, England, on 18 February 1880, the son of Eliza Maria Sandford and her husband, Archibald Sinclair, a shipping merchant. His parents later divorced, and after his mother married George Burgess, a Congregational minister, William was given the surname Burgess. In the early 1890s the family emigrated to New Zealand, where his stepfather became minister of Congregational churches in Auckland and Timaru.
William worked in New Zealand as a carpenter and engineer and joined the Volunteer Force. After serving two years in the South Canterbury Mounted Rifles in Timaru, he transferred to the New Zealand Regiment of Field Artillery Volunteers in Auckland in 1902 and rose to captain in 1909. He joined the New Zealand Permanent Forces in 1911 as one of the first officers of the New Zealand Staff Corps and became adjutant of the 16th (Waikato) Regiment and officer commanding No 4 Area Group in Hamilton.
On 4 November 1913 Burgess went to Tasmania on exchange with the Commonwealth military forces. During the First World War he fought with the Australian Imperial Force in the Middle East and France. At Gallipoli he was major commanding 9th (Tasmania) Battery of the 3rd Field Artillery Brigade of the 1st Australian Division. He was wounded in May 1915. For his leadership of 9th Battery on the right flank of the Anzac position from May to October (when he was invalided to Egypt), he was made a DSO. Promoted lieutenant colonel in March 1916, he went to France commanding the 3rd Field Artillery Brigade. In 1917 he rose to brigadier general and became artillery commander of the 4th Australian Division until the end of the war. He was mentioned in dispatches six times and decorated by the French and United States governments. In 1919 he was invested as a CMG and CB.
Returning to New Zealand in 1919, Burgess was marked for accelerated promotion. As a lieutenant colonel he was senior artillery officer at Wellington Military District headquarters, Palmerston North, until 1922, when he went to General Headquarters as general staff officer, first grade. In January 1924 he became director of military intelligence and training, and in April, chief of the general staff with the rank of colonel. In 1926 he announced that, for family reasons, he was changing his surname to Sinclair-Burgess. He had used the name, however, five years earlier when on 26 June 1921 he married Flora Macdonald Pembroke (née Lowe) in London.
Sinclair-Burgess was promoted brigadier in 1928, and during a visit to Britain observed motorisation of transport and the famous experimental armoured formation on Salisbury Plain. In 1931 he was promoted major general on receiving the dual appointment of general officer commanding and chief of general staff, New Zealand military forces. Depression economies had caused the abolition of compulsory military training so he had the task of reorganising the Territorial Force on a voluntary basis.
He also had to confront the new strategic situation induced by the Japanese attacks on Manchuria in 1931, and Shanghai in 1932. He persuaded the government to set up a defence requirements committee, which reported on 28 August 1933, recommending a six-year plan for rearmament. Suggesting that New Zealand should prepare to defend itself in case the Royal Navy could not fulfil the 'main fleet to Singapore' strategy in a war, his committee recommended new coastal and anti-aircraft guns for Auckland and Wellington; the acquisition of Vildebeeste aircraft for reconnaissance and escort; preparations for an expeditionary force of an infantry division and a mounted rifles brigade; the creation of a regular infantry battalion and an artillery battery to get experience with the British Army in India; and the formation of a New Zealand squadron in the Royal Air Force. For higher co-ordination, he proposed a New Zealand section of the Committee of Imperial Defence (CID). Although the government went ahead with the CID and the Vildebeeste and artillery purchases, the other aspects of the plan were shelved.
Sinclair-Burgess was knighted in 1934, and in 1935 his appointment was extended for two years. In 1936 the first Labour government appointed John A. Lee to review the armed services. This led to extensions at Devonport dockyard to accommodate larger cruisers and the creation of a separate air force to be equipped with new air bases and Wellington bombers. The army then became the Cinderella of the services, but Sinclair-Burgess conducted some well-publicised experiments at Waiouru in 1936 for motorising the army to give greater mobility. In the Second World War he offered his services to the government, and General Edward Puttick wanted him as brigadier at General Headquarters. His offer was turned down, but it is said that he worked for the security services.
Major General Sinclair-Burgess was known as 'Sinky-Boo' after an army version of The mikado. Described by Major General W. G. Stevens as a 'tall dark man with flashing eyes and a striking appearance, with a touch of flamboyance, and a love of self-publicity and self-display', he particularly cherished his full-dress uniform, which he wore to cabinet meetings. This may not have endeared him to Labour ministers. Bob Semple once greeted him with, 'Feathers and all to-day, eh General?'. He also had a fetish about secrecy, but was known to be kind and approachable. Using his carpentry skills he had built his own house at Mahina Bay near Wellington. When it burnt down in 1959, with the loss of all his papers and belongings, army officers subscribed to replace his medals and insignia.
William Sinclair-Burgess died in Lower Hutt on 3 April 1964. He and his wife had divorced in 1928 and there were no surviving children. He was remembered as the most decorated and longest-serving head of the army, the one who held it together during the worst years of the depression.