Enlisting in the Royal Australian Air Force in November 1940, he briefly served in the European Theatre before transferring to Malta.

Over the next five months, Brennan was officially credited with the destruction of 10 Axis aircraft from a total of twenty-four operational sorties.

Reposted to England, he was assigned as a flying instructor and collaborated in the writing of Spitfires over Malta, a book about his experiences on the island.

After leaving school, Brennan studied part-time at the University of Queensland, while simultaneously being employed as a law clerk in Brisbane.

During this time, he was advanced to temporary flight sergeant on 4 January 1942, prior to receiving a posting to the Mediterranean Theatre the following month.

Flying Supermarine Spitfires, the party took off from the aircraft carrier HMS Eagle; they were to spend the next few months in the defence of the island.

[1] On 20 April 1942, Brennan added a further two aircraft to his tally when he destroyed a Messerschmitt Bf 109, before bringing down a Junkers Ju 88 later that day.

[3] The announcement and accompanying citation for the award was published in a supplement to the London Gazette on 22 May 1942, reading:[4] Air Ministry, 22nd May, 1942.

[5] The following day, German Luftwaffe General Albert Kesselring ordered a contingent of 20 Stukas and 10 Junkers Ju 88s with a small escort of fighter aircraft to bomb Grand Harbour, Malta.

At approximately 14:00, Brennan landed his Spitfire in the wake turbulence of the aircraft ahead of himself and touched down on the left side of the runway.

[1][11] Described as one with "an easy-going nature, an engaging sense of humour and ... loyal to his friends",[1] Brennan was buried in Townsville War Cemetery.