Mallaby's death became a major event of hostilities in Surabaya surrounding Indonesian Independence, triggering a retaliatory military action by British forces in the city.
Mallaby was born to William Calthorpe and Katharine Mary Francis Mallaby on 12 December 1899.
He attended the Wellington Cadet College in India and was commissioned as a Second Lieutenant on the Unattached List, Indian Army on 1 October 1918, being admitted to the Indian Army on 8 October 1918 and was appointed to the 27th Punjabis.
He transferred to the 67th Punjabis (later 1st battalion, 2nd Punjab regiment) on 20 July 1919 and promoted lieutenant on 1 October 1919. He served in Waziristan between 1921 and 1924. He received promotion to captain on 1 October 1924.
He attended Camberley Staff College between 1930–1931 and was appointed a General Staff Officer, 3rd grade on 5 February 1933, being promoted to General Staff Officer, 2nd grade (for RAF co-operation duties), on 1 April 1933. This appointment lasted until 4 February 1937. He was promoted brevet major on 1 July 1935 and substantive major on 1 October 1936.
He was appointed a General Staff Officer, 2nd grade at the War Office in London on 1 March 1938 (being promoted war substantive Lt-Col on 15 August 1941) later being appointed Deputy Director of Military Operation in the rank of Brigadier and remained at the War Office until 1942.
He was appointed an Officer of the Most Excellent Order of the British Empire OBE as Major, temporary Lt-Col, acting Brigadier.
He returned to India and served as Major and second in command of the 6th battalion, 2nd Punjab Regiment, from April to August 1943. In August 1943 he was given command of a battalion of the Hyderabad Regiment but after only 6 weeks in command he was appointed Director of Military Operations at G. H. Q. India with the acting rank of Major General.
He was promoted from acting to temporary Major-General and war substantive Colonel on 1 March 1944. In order to obtain operational experience he dropped in rank to temporary Brigadier in July 1944 and was given command of the 49th Indian Infantry Brigade, then serving with the 23rd Indian Division. His substantive (permanent) rank was advanced to lieutenant-colonel in October 1944.
He was appointed a Companion of The Most Eminent Order of the Indian Empire in the London Gazette of 1 January 1945 as a temporary Brigadier.
He led the 49th Indian Infantry Brigade to Indonesia in the middle of the national revolution, to find and repatriate former Japanese prisoners of war. He arrived in Surabaya on 25 October 1945. Upon landing, he sent Captain Douglas MacDonald to contact the leader of the revolutionaries, Moestopo; Moestopo stated that they would not oppose the British forces. Mallaby and his squadron worked under constant supervision by the Indonesians, whom he later told that he was focused on finding the POWs.
However, the situation became more heated on 27 October after Mallaby interpreted pamphlets demanding the immediate surrender of the Indonesians' weapons, signed by General Douglas Hawthorn, as an order. Communications were broken between Mallaby's forces and the Indonesians, and the following day the Indonesians began launching attacks on the 49th Brigade. To quench the fighting, Mallaby was able to contact General Hawthorn through an intermediary and arrange a meeting between himself and President Sukarno, where they negotiated a ceasefire.
Mallaby was killed on 30 October 1945. At the time, he was travelling about Surabaya under a white flag to spread the news about the cease fire agreement and rescue some stranded Maratha troops, despite being warned of the danger by Force 136 troops. When his car approached the British troops' post in the International building near the Jembatan Merah ("Red Bridge"), his car was surrounded by Indonesian Republican militia. Fearing that their commander was about to be attacked, the British troops in the International building, led by Major Venu K. Gopal, fired into the air to disperse the Indonesian militia. The militia, thinking that the British were taking hostile action, fired back at the British troops.
Captain R.C. Smith, who was in the stationary car, reported that a young Republican shot and killed Mallaby after a short conversation. Smith then reported throwing a grenade from the car in the direction of where he thought the shooter had hidden. Although he was not sure whether or not it hit its target, the explosion caused the back seat of the car to ignite. Other accounts, according to the same source, stated that it was the explosion and not a shooter that killed Mallaby.
Whatever the exact circumstances of his demise, Mallaby's death was a significant turning point for the hostilities in Surabaya, and a catalyst for the battle to come. The British ordered an Indonesian surrender, and on 10 November they rolled out a large retaliatory attack. His death also caused the British command to lose trust in the Indonesian politicians.
He received a posthumous Mention In Despatches in April 1946 as a Temporary Brigadier.
Mallaby married Margaret Catherine Jones (known as Mollie) on 9 April 1935 at St Mark's Church, North Audley Street, London. Mallaby is buried in the Commonwealth War Graves Commission Cemetery in Menteng Pulo, Jakarta. His son, Sir Christopher Mallaby, was later British Ambassador to Germany and to France.
Patrick Heren, writing for Standpoint, notes that Mallaby was considered a "thinking soldier" who preferred discourse over fighting.