He served with an Australian Militia unit, the 8th Field Artillery Brigade, from February 1933, and became a clerk in the Crown Law Department of the State Public Service later that year.

Hannah accompanied the squadron, which operated Hawker Demons and Avro Ansons, to its new location at the recently opened RAAF Station Pearce, Western Australia, in March 1938.

[5][6] Promoted to flight lieutenant, Hannah was posted to Britain in July 1939 to undertake a Royal Air Force armaments training course, which he had barely begun when war was declared on 3 September.

[10] In September 1944, Hannah was appointed senior air staff officer (SASO) at Headquarters Western Area Command, Perth.

[1][6][14] Promoted to substantive group captain in October 1950,[15] Hannah was appointed an Officer of the Order of the British Empire (OBE) in the 1951 New Year Honours, in particular for his "exceptional ability" as SASO at RAAF Overseas Headquarters.

As aide-de-camp to Queen Elizabeth II, Hannah was heavily involved in planning the RAAF's part in the 1954 Royal Tour of Australia.

[1] He was posted to Singapore as SASO, RAF Far East Air Force Headquarters, in January 1956, handling counter-insurgency operations during the Malayan Emergency.

[1] He was later described by his staff officer in this role as "brusque" and "impersonal" though not unsympathetic, his "uncommunicative" manner stemming from a preference to "do his own research, think out the substance of his project submissions, dictate to his stenographer, then amend to his own satisfaction", rather than delegate.

Hannah fundamentally disagreed with any suggestion that the Royal Australian Navy should operate land-based aircraft, claiming that he was arguing not from a partisan perspective but to ensure that Australia's limited defence resources were not spread across three services.

Confidential RAAF papers from the time declared that its goal was always to "avoid giving the Navy the opportunity to establish a land-based air force".

[28] Two years later, Hannah responded favourably to a recommendation from the Chairman of the Chiefs of Staff Committee, Admiral Sir Victor Smith, to use the soon-to-be-delivered F-111 bomber for maritime support, among other roles.

One of Hannah's successors as CAS, Air Marshal Errol McCormack, ordered that the uniform revert to Williams' original colour and style commencing in 2000.

[33] Hannah's planned three-year term as Chief of the Air Staff was cut short by some ten months when he accepted an offer to serve as Governor of Queensland, becoming the first officer in the RAAF to receive a vice-regal appointment.

[35] Hannah did not have a strong connection with Queensland at the time of his appointment, and had only lived in the state during his period as commander of RAAF Station Amberley between 1949 and 1951.

[36] He claimed not to have actively sought the governorship, and was criticised for failing to consult with senior colleagues before making his decision to retire early from his position as head of the Air Force.

In October that year, he created controversy at a Brisbane Chamber of Commerce luncheon by criticising the "fumbling ineptitude" of Prime Minister Gough Whitlam's Federal Labor government for placing Australia in "its present economic state".

[1][38] Vice-regal appointees in Australia are expected to remain neutral and above politics but Hannah declared that he would be "guilty of sheltering behind convention, of denying my heritage and failing in my regard for the people of Queensland" if he did not speak his mind.

[41] The incident occurred in the midst of a constitutional crisis and, according to military historian Chris Coulthard-Clark, was "widely seen as a blatant intervention in the national political arena".

Hayden observed that, "Hannah's transgression was not so much that he suffered from an excessive notion of his own importance, which he did, but rather in the intemperate manner he assailed the national government at a public function in Brisbane.

"[43] Following his succession in November 1975, Liberal Prime Minister Malcolm Fraser attempted to have the dormant commission reinstated, but the Queen – following advice from the British government that cited Hannah's lack of impartiality – refused her assent.

[38][44] On 9 October 1976, Hannah dedicated a memorial at Cairns to commemorate the crews of RAAF Catalina flying boats who lost their lives in the South West Pacific during World War II.

[5] He was appointed a Knight Commander of the Royal Victorian Order (KCVO) in August 1977 (backdated to March) as part of Queen Elizabeth II's Silver Jubilee visit to Australia.