Arnold Fielder Downer was born on 4 February 1895 at Alexandra, Victoria, Australia, the son of Edwin Harry Downer, a builder, and his wife, Mary Ann McMinn. The family emigrated to New Zealand in 1899 and settled in Feilding. After attending Manchester Street School and the Feilding District High School, Arnold went to Wanganui Technical College.
In 1913 he joined the Public Works Department as a cadet and worked from the Dunedin offices until he enlisted in the New Zealand Expeditionary Force in September 1915. He served in Egypt with the Corps of New Zealand Engineers, and in France with the New Zealand Field Artillery; he was commissioned from the ranks. He was later in the army of occupation in Germany. In early 1919 he studied civil engineering in London, and he continued his studies after returning to New Zealand later that year.
Rejoining the Public Works Department, Downer worked in Hawke's Bay, Northland and Wellington, and in 1927 he was appointed engineer in charge of the Wellington–Tawa Flat deviation of the main trunk railway line. These major works included the driving of some three miles of tunnels – valuable experience for Downer, whose subsequent career was largely centred on such work. He left the Public Works in 1930 to work with the Hansford and Mills Construction Company, the firm contracted to build Wellington's Mount Victoria road tunnel. Downer was engineer in charge of tunnelling, but when the contractors went into liquidation he was appointed manager for the receiver.
When the tunnel was completed in 1931 he formed a syndicate with George McLean, who had supplied some of the plant. However, they failed to gain any worthwhile contracts and in July 1933 Downer set up his own business, Downer and Company. Three of McLean's sons were to become prominent in the firm. The first successful tender by the new company was for construction work at the Waipori hydroelectric works for the Dunedin City Council. Tunnelling was again a key element, and the contract led to similar work at Cobb River and the Homer Tunnel.
As the company's workload grew, Downer quickly recognised the potential of powerful crawler tractors and associated equipment for earth moving. This machinery was used to build airfields in New Zealand and was put to the same use in the Second World War in the Pacific islands. After the war, Downer moved the company into opencast coalmining and quarrying. He also employed heavy earth-moving equipment in the innovative construction of several earth dams in the Waitakere and Hunua ranges for Auckland's water supply.
In the late 1940s Downer was among the first to use the system of joint-venture contracts, under which two or more independent contractors could hold the contract for the same project. His first such venture was for the lower Nihotupu dam with Papakura-based W. Stevenson and Sons. A much larger contract, with the Morrison-Knudsen Company of the United States, was for building the 5½-mile Remutaka rail tunnel in the early 1950s. New methods employed in the hard rock of the Remutaka Range were to influence tunnelling practice in New Zealand over the following decade. The construction of the Remutaka tunnel, and of the Roxburgh hydroelectric works, built with the government and overseas concerns in the mid 1950s, were the pinnacle of Downer's professional career. Although there was resentment in some quarters that overseas contractors had secured prestige contracts, Downer and Company's involvement in the projects countered some of this feeling.
In 1954 the company became a subsidiary of William Cable Holdings. William Cable had already merged with A. & G. Price and by 1964 the firm was listed as Cable, Price, Downer. Downer retired as managing director of Downer and Company in 1962 and was a director of Cable, Price, Downer until 1970. He was made a CBE in 1956, and was president of the New Zealand Institution of Engineers in 1959–60. In his retirement he spent three years on the council of the DSIR. Outside his professional interests he was a keen golfer and belonged to the Rotary Club of Wellington.
Downer had been married in Sydney on 14 September 1927 to Phyllis Lorimer Massey. The couple had no children, but brought up a young nephew of Phyllis's following the death of his mother. Phyllis had trained at the Sydney Conservatorium and had commenced a career as a concert singer before her marriage. Along with the wives of other directors, she tried to ensure the well-being of company staff working on remote sites.
Arnold Downer died in Wellington on 16 July 1984, survived by his wife and nephew. A genial, unassuming man, he had always believed that people rather than the works he had been associated with were the key to his success. He placed a high priority on matters relating to his staff and strikes were rare within the company. Fittingly, the title of his presidential address in 1960 was 'Human engineering'.