In 1925, the New Zealand government arranged for a cadetship for a New Zealand student at the Royal Military Academy in England. Weir was the successful applicant, with his experience as a school cadet and as a territorial a factor in the decision by the military authorities to award him the cadetship. He completed his studies and was commissioned as a second lieutenant in the New Zealand Artillery in 1927. He was seconded to Royal Artillery units while in England and returned to New Zealand the following year. For the next few years he served in a number of posts around the country working with Territorial units. In 1931, now a lieutenant, he was among the military personnel dispatched to Napier to assist the local populace after the earthquake there. Two years later he was posted to Auckland where he became adjutant of the 1st Field Artillery Brigade. He received a promotion to captain in 1935 and married the next year.