Robert Forrest was born in Burntisland, Fife, Scotland on 7 February 1865, the oldest child of William and Anne Forrest. The 1881 Scotland census finds Robert living in Leith, Midlothian, with his parents, two brothers, and two sisters, his occupation given as apprentice weaver.
After briefly serving with the 1st Midlothian Volunteer Cavalry, he attested to the 5th Princess Charlotte of Wales Dragoon Guards at Edinburgh on 23 August 1883, reporting his birthday as 23 August 1864, making him 19 years-old. As stated in the 1881 Queen’s Regulations, “The minimum age of a recruit will be the physical equivalent of 19 years of age”; it appears his age was falsified so Forrest could enlist.
Forrest was appointed Lance Corporal on 12 February 1886, promoted Corporal on 20 February 1886, appointed Lance Sergeant on 13 October 1887, and promoted Sergeant on 11 July 1888. He married Annie Thornton on 6 January 1892 in Farnham. Forrest was promoted to Squadron Sergeant Major when his regiment was posted to India on 6 September 1893.
The 5th Dragoon Guards were stationed in India when they were ordered to proceed to South Africa on 8 September 1899. Although the Boer War did not officially begin until October, it was apparent for some time that hostilities were inevitable. The 5th Dragoon Guards were among the first regiments to arrive, with D Squadron landing on the 11th October and taking part in the charge at Elandslaagte (21 October 1899). The remainder of the regiment was delayed because of cases of anthrax discovered among the horses and did not land until the 26th of October. It was on the 30th of October when Lieutenant John Norwood received the only Victoria Cross ever awarded to the 5th Dragoon Guards for rescuing a wounded soldier under heavy enemy fire.
The regiment retreated to Ladysmith as part of the force under the command of General Sir George White where they were besieged from 3 November 1899 until relieved on 28 February 1900. During this time, the regiment made a number of sorties from Ladysmith but many of their casualties were due to illness.
After Ladysmith, the 5th Dragoon Guards were deployed in units of squadron size or smaller protecting the lines of communication. It was while thus deployed that Forrest was slightly wounded in action when he was shot in the forehead during operations at Pimple Hill, Ingogo, Natal on 5 September 1900.
Promoted to Regimental Sergeant Major when RSM Henry Boag was commissioned into the South African Constabulary on 27 February 1901, Forrest served as the regiment’s RSM for the remainder of the war until posted back to India along with the rest of the regiment on 6 April 1902. For his service, Forrest received the Queen’s South Africa Medal with 3 clasps and the King's South Africa Medal with 2 clasps.
While serving in India, Forrest was awarded the Long Service & Good Conduct Medal without gratuity on 1 January 1903. Forrest’s timing was fortuitous. The appointment of Sergeant Major conferred the rank of Warrant Officer, and Warrant Officers were ineligible for the LS&GC from July 1882 to June 1902. Thus, Forrest became one of the first RSMs to receive this medal after the statute was revised.
Forrest returned to South Africa on 15 March 1904 with his regiment before being sent home on 28 December 1908.
Having reached the mandatory age, Forrest was discharged from the 5th Dragoon Guards on 22 August 1909 after 26 years of service, his conduct being described as “exemplary”. His intended place of residence was listed as 32 Bothwell Street, Edinburgh.