McMullen was born in Summer Hill, New South Wales, on 27 August 1908,[1] the son of Frank and Blanche MacMullen,[2] and was educated at Fort Street High School before completing a degree in veterinary studies at the University of Sydney.

[6] McMullen moved from Australia to the United Kingdom in 1931 to continue flight training, and received a temporary commission as an officer in the Royal Air Force (RAF) shortly afterwards.

[9][10] McMullen was responsible for encouraging Don Bennett to try the de Havilland Mosquito, which proved crucial for the effective use of the Oboe target finding device.

[11] McMullen was promoted acting wing commander on 11 March 1943,[12] and awarded an Air Force Cross in the 1943 New Year Honours.

[14] As the commanding officer of RAF Bardney he led the deployment of two squadrons of Avro Lancaster bombers to the Soviet Union in September that year during Operation Paravane, an attack on the German battleship Tirpitz in northern Norway.