William Slacum joined the US Navy in the summer of 1829 at the age of 30. Two years later, he served as the purser aboard the USS Potomac. A naval purser served as a supply and financial officer aboard ships. He acted both as the maintainer of ship’s pay and muster roles but also ran a ship’s store from which a sailor’s pay would be deducted for articles of clothing or luxury items like tobacco, sugar, tea or coffee. Purser positions were highly sought after. The title of purser in the American navy would change in 1860 to paymaster. Today, they men and women officers belong to the Pay Corps of the navy.

Aboard the Potomac, he spent four years circumnavigating the world. In December 1833, Slacum found himself posted as Special Agent of the Pacific Squadron in Valapraiso, Chile. This gave him time to recover from a bout of trigeminal neuralgia – tic douloureux. His posting was subject to confirmation from Washington, which he failed to obtain when the Navy appointed someone else to fill the post.