After schooling at Norwood Public School, Weir joined the South Australian Lands and Survey Department. At 19 he enlisted in the Volunteer Military Force and in 1890 was commissioned in the South Australian Militia. He achieved the rank of colonel and was made commander of the 19th Infantry Brigade in 1913.

At the outbreak of the First World War in 1914, the 10th Infantry Battalion was raised in South Australia. Weir was given command of the battalion with the rank of lieutenant colonel in the Australian Imperial Force (AIF). The battalion left South Australia in October 1914 and arrived in Egypt in early December. After training in Egypt and Lemnos, Weir's battalion was one of the two AIF units to land at Gallipoli Cove on 25 April 1915. Approximately half of the battalion was killed at Gallipoli. Weir became ill in September 1915 and was evacuated before the withdrawal of the Australians from Gallipoli in December.

Weir rejoined his men in Egypt and the battalion embarked for the Western Front in March 1916. After seeing heavy fighting at the Somme in July and August Weir was relieved and returned to Australia, still in poor health. He retired from the AIF in December 1916. In January 1917 Weir was awarded the Distinguished Service Order and was mentioned in Despatches for distinguished and gallant services and devotion to duty in the field. The following month he was awarded the Order of St Anne, 2nd Class (with swords) by the Tsar of Russia.

From 1917 to 1931 Weir was South Australia's first public service commissioner. He was an active member of the Church of Christ, and served as president of the church conference in 1924. Throughout his working life and after his retirement Weir played an important role in charitable and welfare activities in South Australia. He served as chairman of the Children's Welfare and Public Relief Board and the Central Board of Health, president of the Cheer-Up Society, Our Boys' Institute and the YMCA, general secretary of the Adelaide Benevolent and Strangers' Friend Society and was a member of several service oriented organisations.

August 1914: Made commander of the 10th Infantry Battalion; the first South Australian officer to receive a commission in the Australian Imperial Force (AIF)

21 December 1916: Appointed South Australia's first public service commissioner

1 January 1917: Awarded the Distinguished Service Order

4 January 1917: Mentioned in Despatches

15 February 1917: Awarded the Russian Order of St Anne, 2nd Class (with swords)

28 April 1921: Made Brigadier-General on the retired list

Weir was the first South Australian officer to receive a commission in the Australian Imperial Force (AIF).

'Brig.-Gen. Weir dead: Notable career of public service', The Advertiser, 15 November 1944, p. 4, col. k

Lock, Cecil Bert Lovell. The fighting 10th : a South Australian centenary souvenir of the 10th Battalion, A.I.F., 1914-1919, Adelaide : Webb & Son, 1936

Radbone, Ian and Jane Robbins. 'The history of the South Australian public service', The Flinders history of South Australia. Political history, Dean Jaensch (ed.), Netley, S. Aust. : Wakefield Press, 1986, pp. 458-60, p. 462

Australian War Memorial See: Stanley Price Weir and 10^th Battalion

National Archives of Australia: Search RecordSearch for Weir, Stanley Price to find view a digital copy of Price's service records

South Australians at War [State Library of South Australia]