John Evelyn Duigan was born at Whanganui on 30 March 1883, the son of Mary Emily Broad and her husband, James Duigan, a miller and later editor of the Wanganui Herald. He attended Wanganui Collegiate School in 1894, but the extent of any further schooling remains obscure. Following the outbreak of the South African War in 1899, he worked his passage there and enlisted in Brabant's Horse, a Cape Colony volunteer unit. Within a month he was seriously wounded at the siege of Wepener. He was invalided home in December 1900 after his horse fell on him, but went back to South Africa; he was wounded again and rose to the rank of sergeant. Soon after returning to New Zealand in February 1902, he secured a commission in the Tenth Contingent, which left Wellington on 14 April 1902 but arrived in South Africa too late to see action.
Having resolved to make a career in the military, Duigan was appointed a cadet in the New Zealand Permanent Force in 1903, and in 1905 he was commissioned as a lieutenant in the Royal New Zealand Engineers. The following May he went to England to attend a course on field engineering and electric lighting, later also attending one on signalling. After his return to New Zealand in July 1908, he was employed as an instructor in signalling and engineering at Defence Department Headquarters in Wellington. He was transferred to the New Zealand Militia soon afterwards.
Duigan was married in Wanganui on 26 April 1909 to Norah Hanley; they were to have three sons. In January 1911 Duigan was transferred to the New Zealand Staff Corps. Early in 1913 he began a two-year course at the Staff College at Quetta, India, where he demonstrated 'superabundant energy, great determination and dogged perseverance'. Following his graduation in October 1914, he was appointed general staff officer in the New Zealand Expeditionary Force's headquarters staff, but nervous exhaustion led to his being sent to England on leave early in 1915. He returned to New Zealand in June to help fill a shortage of staff officers.
Duigan was appointed to command the New Zealand Tunnelling Company, which was raised late in 1915 and included among its ranks many skilled miners. He claimed the strain of combat service in France was heightened by the difficulties arising from the presence of '17 ex-secretaries of Labour Unions in the Unit', as well as members of the Red Federation.
After serving briefly as a staff officer at New Zealand Divisional Headquarters, in February 1917 Duigan assumed a staff position at 6th Corps headquarters, and in the second half of the year he was brigade major of the New Zealand Infantry Reserve Group. During that year he was mentioned in dispatches and made a DSO. In December 1917 he became an instructor at the Senior Officers' School at Aldershot, England, but in March 1918 he was sent on a several-months-long propaganda tour to the United States to promote the sale of Liberty loan bonds. He was repatriated to New Zealand in October 1918.
Resuming his career in the New Zealand regular force, Duigan was briefly chief infantry instructor at Featherston Military Camp before being appointed, early in 1919, to a staff position in the Auckland Military District (later Northern Command). He was noted for his ability as an instructor and lecturer. In October 1930 he assumed charge of the Northern Command, and in 1931 was promoted to the rank of colonel.
Duigan was appointed general officer commanding and chief of the general staff on 1 April 1937, the first New Zealand-born soldier to hold the army's highest position. He was also promoted to major general and made a CB. His first task was to reorganise the military forces, the more effectively to reflect the numbers available under the voluntary system of recruitment. His proposals included the abolition of the position of general officer commanding and the institution of control by an army board. Duigan became first military member of that board in November 1937.
The reorganisation upset many Territorial Force officers, and led four colonels to breach King's Regulations by publishing a manifesto decrying the weakness of the army, for which they were posted to the retired list amidst much publicity. Duigan's somewhat casual approach left some observers unconvinced by his performance. Although he faced great difficulties because of the government's steadfast refusal to consider conscription and its predilection for air force development, it was felt he could have done more to overcome the political obstacles to improving the army.
When the Second World War broke out, Duigan aspired to command the expeditionary force which the government decided to raise as part of New Zealand's war effort. But, despite his amiable qualities, Duigan's lack of experience as a commander in the field and doubts about his intellectual capacity and temperament told against him. His appointment would have been unpopular in the army, and was never seriously considered by the government, which instead appointed Major General Bernard Freyberg. A disappointed Duigan was offered an extension of his term as compensation. He was knighted in 1940. Until his retirement on 31 May 1941, he effectively supported 2NZEF, which he visited briefly in the Middle East in late 1940 after attending conferences in Singapore and New Delhi. During this tour he also spent a short period in Greece, where some New Zealand troops were deployed.
Norah Duigan had died on 23 August 1940, and on 6 March 1947, in Wellington, Duigan married Dorothy Violet Tate (née Butt), a widow. John Duigan died at Havelock North on 9 January 1950, survived by his wife and two sons of his first marriage. His third son had been killed in Canada while serving with the Royal Air Force during the war.