William Douglas Weir was born in Glasgow on 12 May 1877, the son of James Weir of Over Courance, Dumfriesshire and Mary Douglas of Kilmarnock. He was educated at Allan Glen's School and Glasgow High School, and entered the family engineering firm of G & J Weir Ltd, where he served an apprenticeship in all departments of the business. He became a Director in 1898 at the age of 21, Managing Director in 1902 ; and succeeded his father as Chairman in 1912. In 1904 he married Alice Blanche, daughter of John Machonnachie of Glasgow. They had two sons and one daughter.
Weir had a great interest in motor cars which led to his association with the French based motor manufacturing company managed by Alexandre Darracq, and to G & J Weir Ltd becoming involved in development work on steam driven buses. This venture was largely unsuccessful, and in 1912 he severed the connection with Darracq. His interest in motor cars continued with his involvement in the Royal Scottish Automobile Club of which he had been a founder member, later becoming President. With the outbreak of World War I Weir began to look for ways in which his company could become involved in the war effort. The company became involved in the production of shells, setting up factories at their premises in Cathcart, Glasgow. In July 1915 Weir became Scottish Director of Munitions for the Government's Ministry of Munitions. The shell production work was transferred to Cardonald in 1917, leaving space at Cathcart for the introduction of aeroplane manufacture. The company became the largest Clydeside producer of military aircraft. Although he remained Chairman of the company, 1917 was also the year in which Weir went to London as Controller of Aeronautical Supplies and a Member of the Air Board. He went on to serve as Air Minister (1918) then Secretary of State for Air Forces and President of the Air Committee until 1919. He was knighted in 1917 and created a Baron and Privy Councillor in 1918. He was appointed Chairman of the House of Lords Advisory committee on Civil Aviation in 1919.
Soon after peace was declared Weir returned to Glasgow to resume his duties as Managing Director of the family firm. The company moved out of aircraft production and back to their pre-war focus on the manufacture of auxiliary machinery and pumps, but the inter-war years were very difficult and the company was forced to diversify into housing and the production of a nickel-copper alloy for use in the production of propellers and tubes. Throughout this period Weir continued as a Government advisor on Civil Aviation and the RAF but the build-up to world War II saw this involvement increase significantly again. In 1934 he was admitted to the Order of the Bath, and was created a Viscount in 1938. With the outbreak of World War II he was appointed Director General of Explosives at the Ministry of Supply, a post which he held from 1938-1941. He became Chairman of the Tank Board in 1942. After the war the company's earlier interest in housebuilding was revived, first with steel houses and later with more conventional types of house building, this division of the company flourishing alongside the pump manufacture. Lord Weir retired as company Chairman in 1954 and became Honorary President the following year.
Throughout his life he was active in professional bodies, including the Institute of Marine Engineers, the Institution of Naval Architects, and the Institution of Engineers and Shipbuilders in Scotland. His other appointments included Industrial Adviser to the British Delegation at the Imperial Conference at Ottawa in 1932; Honorary President of the British Employer's Confederation; and Director of the International Nickel Company of Canada Ltd. In addition to his UK Honours, he received a number of other national awards, including the Order of the Crown of Italy; the American DSM; and the French Commander of the Legion of Honour. The University of Glasgow awarded him the Honorary Degree of LLD in 1919, and he was awarded the Freedom of the City of London in 1957. His public life in Glasgow extended to his becoming a JP and Deputy Lieutenant for Glasgow and Renfrewshire. His home was at Eastwood Park, Giffnock, where he died on 22 July 1959.