Gifted both academically and athletically, Mulgan studied at Auckland University College, before attending Merton College, Oxford. He was awarded a first in English in 1935 and subsequently took up a position at the Clarendon Press.

Mulgan held leftish political views and was alarmed by the rise of fascism in Europe and the response of the British government to it. In 1936, he was an observer for the New Zealand government at the League of Nations in Geneva. During this time, he wrote a series of articles on foreign affairs, titled "Behind the Cables", for the Auckland Star newspaper.

His view that war in Europe was inevitable, led Mulgan to join the Territorial Army in 1938, and he was made second lieutenant in an infantry regiment.Posted to the Middle East in 1942, Mulgan was promoted to major and made second in command of his regiment. He saw action at Alamein and fought alongside the New Zealand Expeditionary Force. He was impressed by the calibre of his compatriots and found meeting New Zealanders after being in England for so long to be a kind of "homecoming".

In 1943, Mulgan joined the Special Operations Executive and was sent to Greece to coordinate guerrilla action against the German forces. He was awarded the Military Cross for his actions. After the German withdrawal in 1944, Mulgan oversaw British compensation to Greek families who had helped the Allied forces.

In the evening of Anzac Day 1945, Mulgan intentionally took an overdose of morphine. Speculation remains as to why he committed suicide. He is buried at Heliopolis military cemetery in Cairo. Mulgan was survived by his wife Gabrielle (married 1937) and son Richard (born 1940).