Educated at the Royal Military Academy Sandhurst, Forestier-Walker was commissioned into the Scots Guards as Ensign and Lieutenant, by purchase, 5 September 1862, and was appointed a Lieutenant and Captain, by purchase, 11 July 1865. In 1873 he was appointed Military Secretary to the General Officer Commanding Cape Colony and 15 October 1878 was promoted Colonel. He was promoted to Captain and Lieutenant-Colonel of the Scots Guards 20 March 1880.
In 1882 he was appointed Assistant Adjutant and Quartermaster-General for the Home District but shortly after returned to South Africa.
He was appointed a Brigadier at Aldershot in 1889 and Commander of British Troops in Egypt in 1890, during which he was knighted and promoted to a Knight Commander of the Order of the Bath. Upon returning from Egypt in 1895, he was appointed General Officer Commanding Western District, serving until 1890.
In 1899 he again returned to Africa, becoming GOC Cape Colony and acting as Lieutenant-General in command of Lincolnshire of Communication, South Africa Field Force, 1899–1901. He was thus responsible for disembarkation of troops and military stores, and sending them to the front.
In a dispatch dated 31 March 1900, the Commander-in-Chief in South Africa, Lord Roberts, wrote how Forestier-Walker carried out his duties "with cr to himself and with advantage to the public service".
He was Governor of Gibraltar from 1905 until shortly before his death in 1910, and also acted as General Officer Commanding Mediterranean in 1909. In retirement he became a Director of the Cold Storage Company. He was a Knight of Grace of the Order of Street John from 1901.