He was wounded in the landing at Gallipoli on Anzac Day and invalided to Egypt, the United Kingdom, and ultimately Australia, taking no further part in the fighting.

As General Sir Thomas Blamey's principal non-operational subordinate, he was responsible for administering and training the wartime army.

He joined the AIF as a lieutenant on 24 August 1914 and was appointed adjutant of the 12th Infantry Battalion, which was forming at Anglesea Barracks near Hobart.

[2] Northcott embarked for Egypt from Hobart with the 12th Infantry Battalion on the transport A2, HMAT Geelong on 20 October 1914.

On returning to Australia, Northcott served as Staff Officer, and later Director, Stores and Transport, at Army Headquarters in Victoria Barracks, Melbourne.

[1] He was appointed a Member (fourth class) of the Royal Victorian Order on 8 July 1927 for coordinating the transport for the 1927 six-month Royal Tour of the Duke and Duchess of York (later George VI and Queen Elizabeth The Queen Mother) that year to open the Old Parliament House, Canberra.

He returned to England as an exchange officer with the British Army, where he served the staff of the 44th (Home Counties) Infantry Division.

He was one of only six Australian Army officers to attend this prestigious course between 1928 and 1939, the others being Frank Berryman, John Lavarack, Henry Wynter, Vernon Sturdee, Sydney Rowell and William Bridgeford.

[1] For his service as Deputy Chief of the General Staff, he was made a Companion of the Order of the Bath on 1 January 1941.

When the commander of the 9th Division, Major General Henry Wynter, fell ill in January 1941, Lieutenant General Sir Thomas Blamey asked for Northcott to replace him, but Northcott was involved in organising the 1st Armoured Division and the appointment instead went to Brigadier Leslie Morshead.

[2] Northcott was succeeded as commander by Major General Horace Robertson, an officer with a distinguished combat record in the desert.

[11] Northcott spent much of his time from 1943 on in a long battle with the government over the number of men and women allocated to the Army.

In September 1944, the government reduced the Army's monthly intake of women from 925 to 500, while it only received 420 out of 4,020 men allocated to the three services.

The relationship could still have been a delicate one, but in the event it was characterised by none of the rancour and rivalry that marred the wartime administration of the RAAF.

[14] However, when Blamey travelled to Washington, D.C. and London in April 1944, he arranged for Northcott to act as Commander-in-Chief in his absence.

He made it a condition of his acceptance that Northcott be given the appointment of Commander-in-Chief of the British Commonwealth Occupation Force (BCOF) in Japan.

As such, he negotiated the Northcott-MacArthur agreement with General of the Army Douglas MacArthur, which governed the terms and conditions under which the BCOF would occupy part of Japan.

[1] In April 1964, Northcott and Forde represented Australia at General MacArthur's funeral in Washington, D.C.[25] Sir John's wife, Winifred Mary predeceased him on 7 June 1960.

He was accorded a state funeral with military honours and was cremated with his ashes interred with his wife at Northern Suburbs Memorial Gardens.

[26] In 1968 the Northcott Municipal Council, comprising large areas formerly in the City of Sydney, and the Electoral district of Northcott in the New South Wales Legislative Assembly were dedicated in his name (the council was renamed in December 1968, while the electoral district existed until abolition in 1999).

[27] In his military career, Northcott was both highly regarded and successful staff officer, as commander of the 1st Armoured Division, II Corps and BCOF he was "noted neither for innovation nor conspicuous success",[1] especially when compared with Robertson who "possessed the ebullience and flair that Northcott lacked".