Before settling in London, Hassett served in the British Army under Colonel Louis Prescott. They were deployed on the Gold Coast of Africa to fight the Ashanti. At some point during their time overseas, Prescott lead them into a sortie across the Pra River, hoping to outflank the Ashanti warriors. The sortie was a failure, and several of their comrades were killed in action.
Following this, Prescott resigned and started a munitions factory in London. Shortly after, Hassett also left the service, promptly receiving a job offer from his former colonel. While working at the factory, Hassett was a witness to Prescott's murder in 1868. The Assassins Jacob and Evie Frye interrogated him, but Hassett had little information to provide.