He was educated at Westminster School, but early showed a taste for a military life, and in 1788 his father purchased for him a commission as an ensign, and in 1792 the rank of lieutenant and captain in the 3rd Regiment of Foot ( The Scots Guards).
The next year, though only a lieutenant-colonel, he was selected by Sir Ralph Abercromby to be quartermaster-general of his army in the Mediterranean, at the same time that another young Scotsman, John Hope, who was also to gain fame in Moore's retreat, was nominated adjutant-general.
[2] He embarked at Ramsgate in August 1808, and, on reaching the mouth of the Douro in company with Brigadier-General Acland, found orders from General Wellesley to proceed at once down the coast to Maceira Bay.
At the battle of Vimeiro, the churchyard which formed the key of the English situation was occupied by the brigades of Fane and Anstruther, and on them fell the brunt of Junot's attack.
On the arrival of Moore, Burrard, and Dalrymple, the army was re-divided, and Anstruther had the other companies of the 95th given to him, and was put under the orders of Edward Paget, who was to command the reserve.
He presents a singular instance of military devotion; with wealth, domestic happiness, and a certain seat in parliament, he preferred to risk his life and lose it in the service of his country.