[1] Weir served in South Africa between 1899 and 1901 as a non-commissioned volunteer in the Worcestershire Yeomanry and was mentioned in despatches twice and awarded four clasps to his Queen's Medal.

After about six months, he was commissioned as a second lieutenant[1] and transferred into the Regular Army on 11 January 1902 as a captain on probation in the 3rd Dragoon Guards,[2] the appointment being confirmed the following year.

Soon afterwards, in October 1914, he was appointed GSO2 of the newly formed 2nd (Cavalry) Division and in June 1915, he became commanding officer of the 2nd Battalion, Royal Irish Rifles and was slightly wounded in September.

[2] In October, Weir was promoted to the temporary rank of brigadier general[7] and took command of the 28th Division's 84th Infantry Brigade.

[1] Weir was Deputy Lieutenant of Worcestershire from 26 June 1941 and served as Vice-Chairman of the Worcester Territorial Army Association.