Born Hans Ludwig Raimund Koester in 1844 in Schwerin, Mecklenburg-Schwerin, he entered the Prussian Navy as a Kadettenanwärter (Cadet candidate) on 21 June 1859.

Promoted to Korvettenkapitän (Lieutenant-Commander) in 1875, he was later assigned to various training ships, the latest of which was the Kreuzerfregatte (cruiser-frigate) Prinz Adalbert, which he took on a world cruise from 1878 to 1880.

From 1884 to 1887 he served 2.5 years as chief of staff of the German Imperial Admiralty (Kaiserliche Admiralität), then headed by Admiral und General der Infanterie Leo von Caprivi.

When Admiral Alfred von Tirpitz presented his plan for a great battle fleet in June 1897, Koester objected on the grounds that there simply were not the personnel to cover such an expansion of the navy, and that the resources would be much better spent elsewhere.

In 1900, he was instrumental in uncovering the poor preparation and staffing of the German Imperial Admiralty Staff under Diederichs, which "was a threat to national security.

Made an honorary citizen of Kiel on his 70th birthday, 29 April 1914, Koester worked during the First World War as a delegate of the Kaiserliche Marine for health care.