Born in New South Wales, he trained with the Royal Australian Air Force (RAAF) before joining Queensland and Northern Territory Aerial Services (Qantas) as a pilot in 1924.

Seeing little prospect for advancement at Qantas once the war had ended, Brain left to join the fledgling government-owned domestic carrier Trans Australia Airlines (TAA) in June 1946.

After his departure from TAA, Brain became Managing Director of de Havilland Aircraft in Sydney, before joining the board of East-West Airlines as a consultant in January 1961.

[4][5] Brain's penchant for motorbikes and things mechanical inspired a lift driver at CBC to suggest he apply for pilot training in the recently formed Royal Australian Air Force (RAAF).

[6] He was among five civilian students nominated by the Civil Aviation Branch (CAB) of the Defence Department for entry into the inaugural RAAF flying training course, which commenced at Point Cook, Victoria in January 1923.

The benefit of these nominations from a military perspective was that although the destiny of the CAB-sponsored students was to be civil aviators, they would also be members of the RAAF reserve, known as the Citizen Air Force (CAF), and could therefore be called up for active service as and when necessary.

[11] In April 1929, Brain was selected to take part in a search for lost aviators in northern Australia, having gained experience of the area while flying over the Tanami Desert to assist a gold prospecting expedition some years earlier.

[4] On 20 April, he took Qantas DH.50 Atalanta from Brisbane to link up with RAAF Airco DH.9s under the command of Flight Lieutenant Charles Eaton at Tennant Creek, to look for Keith Anderson and Robert Hitchcock in their Westland Widgeon the Kookaburra.

After Brain reported the Kookaburra's position to Eaton, the latter led an overland expedition to the site and buried the bodies of Anderson and Hitchcock, who had evidently survived crash-landing their plane before succumbing to heat and thirst.

[12] His discovery of the Kookaburra and, shortly thereafter, of two lost British aviators in Arnhem Land, earned Brain the Air Force Cross; the award was gazetted on 31 May:[4][13] The KING has been graciously pleased to approve of the award of the Air Force Cross to Mr. Leslie Joseph Brain, in recognition of the distinguished services rendered to aviation by his recent flights in the northern territory of Australia in search of missing aviators.The Gazette later corrected "Leslie" to "Lester".

Travelling via Honolulu, Canton Island and Nouméa, they arrived at their destination after one week, including sixty hours flying time; it was only the third such direct flight to Australia across the Pacific Ocean.

[20] By February 1942, Brain was running the Qantas base at Broome in north Western Australia, which had assumed major importance as a way station for evacuees from the Dutch East Indies, possessing a harbour suitable for flying boats, as well as an airfield that could take heavy bombers.

After the all-clear sounded, he ordered an undamaged Qantas flying boat to Port Hedland, in case of further attacks; he also took part in the search for survivors of a Consolidated B-24 Liberator that had been shot down by the raiders.

He planned to have the first scheduled flights operating by October 1946, around the same time as delivery of four DC-4 Skymaster four-engined liners that would augment the DC-3 fleet, giving the airline a significant edge over ANA.

[24][25] In the event, TAA's first flight, from Melbourne to Sydney, took place on 9 September under pressure from the government, keen to ensure favourable publicity for its new enterprise before the Federal election at the end of the month.

Though praised for contributing to increased civil traffic in Australia, the airline was losing money, generating criticism in media and political circles that it was an inefficient organisation propped up by ordinary taxpayers.

[30] This, plus popular opinion in TAA's favour, helped ensure the airline's survival as a public enterprise in the wake of the Labor government's loss to Robert Menzies' conservative Liberal Party in the Federal election the previous year, though Coles was replaced as Chairman of ANAC by Norman Watt.

Although his departure came as a surprise to ANAC, Brain had for some time felt shackled by having to run TAA on a commercial basis under the control of a government bureaucracy, and on a public servant's remuneration.

[4][32] During Brain's tenure at de Havilland, the company manufactured sixty-nine Vampire T35 jet trainers at its Bankstown factory for delivery to the RAAF, as well as Sea Venoms for the RAN.