Henry George Glas Sandeman was born on 18 June 1868, the younger son of Lieutenant-Colonel John Glas Sandeman, M.V.O., of the King's Body Guard.
He passed the examination for naval cadetships, with 1,002 marks, but did not attain high enough marks to be automatically given a place. In the event he obtained a cadetship because four candidates who had been nominated failed to pass the examination, and another was unable to take his place owing to ill-health.
Sandeman was promoted to the rank of Lieutenant on 1 April 1891.
He was appointed Flag Lieutenant to Vice-Admiral Sir John O Hopkins on 10 November 1896.
Sandeman was promoted to the rank of Commander on 30 June 1901, and then to Captain on 31 December 1906.
He was given command of the battleship Venerable on 14 May 1912, remaining there until he was appointed command of the pre-dreadnought Britannia on 6 December 1913.
He was appointed Commodore-in-Charge of Naval Establishments at Hong Kong on 18 May 1916.
He was appointed an Additional Member of the Third Class, or Companion, of the Most Distinguished Order of Saint Michael and Saint George (CMG) on 1 January 1918.
He left Hong Kong on 18 May that year and was promoted to Rear Admiral on 1 September, and placed on the Retired List on 2 September.
On 28 October, 1918, he was pronounced physically unfit, manifesting cardiac systems unlikely to render him suitable for naval service. In 1922, he requested that he not be promoted to higher flag rank on the Retired List.
Sandeman died at Hayling Island on 16 May 1928 of cardiovascular degeneration.