After completing his education at Christchurch West District High School, he was of the first ten cadets from New Zealand to enter the Royal Military College at Duntroon, Australia, in 1911.

In 1930, he was named commander of a military police contingent that was raised for service in Western Samoa but was ultimately not required.

[5] In 1937 he was a colonel and was part of the New Zealand contingent sent to London for the coronation of King George VI and Queen Elizabeth.

[5] When the 4th Division, one of three intended for home defence, was raised in April 1942 he was made its commander and promoted to temporary major general.

[4] In January 1943, while on a visit to the 2nd New Zealand Division which was positioned on the outskirts of Tripoli, Weir, together with Brigadier William Gentry and Major General Bernard Freyberg, were ambushed and came under fire from Italian and German troops.

Freyberg escaped and organised a party from a machine gun company to retrieve the brigadiers, but both had managed to evade capture by the time of its arrival.

He retired from the military in September 1949, having been appointed a Knight Commander of the Order of the British Empire in the 1948 King's Birthday Honours.