Arthur Leonard Downes was born at Warwick, England, on 27 May 1895, the son of Richard Downes, a fishmonger and game dealer, and his wife, Lavinia Tomlinson. He attended secondary school, then was employed by the Leamington, Warwick, Rugby and County Chronicle as a cadet journalist in 1911. Early the following year he joined the Warwickshire Yeomanry, with whom he served in France in 1914–15 and at Gallipoli in 1915. Transferring to the Hussars, he took part in the Sinai campaign, during which he rose to the rank of squadron sergeant major and was twice wounded. In 1918 he returned to the western front, where he was wounded again.
After the war, from 1920 to 1924, Downes was a territorial soldier in the machine gun corps and served as a civil service clerk in Birmingham, Edinburgh and London. In 1925, while employed by the Department of Overseas Trade, he was sent to Dunedin, New Zealand, as a salesman for the New Zealand and South Seas International Exhibition. The following year he resigned, but remained in New Zealand, taking up various clerical positions. He was a salesman at an Auckland amusement park when he enlisted in the Samoa Military Police in April 1928.
This 74-strong force, which left Auckland for Apia in the Tutanekai on 21 April, had been specially recruited as a response to the challenge mounted by the Mau nationalist movement to New Zealand's rule of Western Samoa under the League of Nations mandate. It had been made necessary by the administration's unavailing efforts to restore order in early 1928, despite the dispatch of the New Zealand Division of the Royal Navy's two cruisers to Apia. Under the direct control of the administrator, Colonel S. S. Allen, the Military Police wore khaki uniforms and were equipped as a military force, but were regarded as policemen rather than as soldiers.
During the next 12 months Downes was involved in a variety of policing duties. There were occasional violent confrontations with the Mau, especially when attempts were made to arrest the leaders. Nevertheless, in an attempt to lessen the military character of the administration's coercive resources, the Military Police were disbanded in April 1929 and replaced by a 50-strong civil police force. Downes, who had proved an effective NCO, was one of 28 military policemen who joined the Samoa Constabulary. He was given the rank of sergeant and appointed a district officer at Falealili.
The unresolved conflict between the administration and the Mau came to a head on 28 December 1929, 'Black Saturday', when the authorities sought to arrest a wanted man conspicuously marching in a Mau procession along the main beach road in Apia. The small arresting party was fiercely resisted, and a fracas developed. Downes was in charge of an 18-man support party armed with revolvers. Watching from a nearby vantage-point he quickly summoned their help. These reinforcements were insufficient to restore order and, fearful for their lives, the beleaguered policemen resorted to using their weapons, firing shots into the crowd which may have killed five people and wounded several others.
Under a hail of stones, the police retreated to the police station in the adjoining Ifi Ifi Street, near which one of their number was cornered and killed by the following mob. As the Samoans approached the police station along Ifi Ifi Street, machine-gun fire was directed over their heads from the station's north balcony by the senior policeman present, Sergeant R. H. Waterson, causing them to fall back. While Waterson was on another balcony deterring Samoans approaching from the east, Downes and two companions, who were manning the north balcony with rifles, panicked and opened fire on the crowd itself. They would say later that the Mau had been threatening to re-enter Ifi Ifi Street from the beach road.
Among several Samoans killed by this fire was the high chief Tupua Tamasese Lealofi III who, probably in trying to restrain the crowd, had made himself a conspicuous target. In dealing with the eight known fatalities (some sources give a greater number) the coroner concluded that the rifle fire had been unnecessary. 'Black Saturday' was the nadir in relations between Samoa and New Zealand. It was followed by a 'state of disciplinary "war" ' as the New Zealand cruisers were again dispatched to Samoa. The Mau were forcibly suppressed and their leaders exiled.
Downes continued to serve with the Constabulary until October 1932. He proved a conscientious officer and made some progress in learning the Samoan language. After returning to New Zealand, he became a member of the New Zealand Police Force in Wellington. He served three years in Masterton, then returned to Wellington in 1940, and was employed in the plain clothes section at Mount Cook Police Station.
On 14 February 1940 at Wellington Arthur Downes married Evelyn Constance McCaull. There were no children of the marriage. That year Downes sought a position in the military forces. Despite police objections because of staff shortages, he was appointed to a commission in the provost section of 8th Infantry Brigade Group in Fiji in November 1940. He returned to New Zealand in August 1942 and was transferred in March the following year from the Second New Zealand Expeditionary Force to the temporary staff. With the rank of captain, he assumed command of the detention barracks at Trentham Military Camp, a post he held until December 1945.
Although Downes resumed his career in the police in Wellington, he hankered after further military service, making unsuccessful attempts to join Jayforce in 1947 and to become a United Nations military observer in Kashmir four years later. He retired from the police on 25 July 1959. Arthur Downes died at Wellington on 1 August 1984, survived by his wife, Evelyn, and was buried in the servicemen's section at Akatarawa cemetery.