Jachmann was born in Danzig and attended grammar school in Marienwerder. He started sea travelling with a trip to the West Indies (1839–1844). From 1844 to 1848, he served on the corvette Amazone in the Mediterranean and at the American coast. In 1845, he was promoted lieutenant. From 1849 to 1852, he commanded a gunboat flotilla at Stralsund before he was transferred to the Prussian Ministry of War. Jachmann resigned when Albrecht von Stosch was nominated Minister for the Navy on December 31, 1871. He died in Oldenburg.