Macnab was born on 2nd December 1870, the third son of Alexander Macnab and Elizabeth Gilpin. He joined the school in 1885 as part of Homeboarders House, and remained for the following two years. Very little is known of his school life, although his role in active military service is honoured in many editions of the Elizabethan running 1899-1918.
He joined the Border Regiment in October 1891 as 2nd Lieutenant, gradually rising up the ranks and becoming Captain of the Northumberland Fusiliers in May 1900. Macnab was then sent to South Africa, serving there for two years, before returning to England in 1903. He continued his military career throughout the early 1900s and became Brigadier General in 1915, one of the highest military rankings. In 1916, he was called to duty in France, where he was mentioned in two separate despatches for his bravery and valour.
While in France, he contracted a serious illness and was forced to return to the UK, whereupon he received the honour of ‘The Most Distinguished Order of St Michael and St George’ as a companion. He married Beatrice Marian in 1918, before passing away on the 13th October that year, as a direct result of illness contracted in service.