He left the relative comfort of his family life at age 18 when he joined the Royal Navy as a mid-shipman with dreams of becoming an officer. However, his hot temper and impulsiveness impeded a swift ascension through the ranks.
Although he was initially hesitant, Duncan quickly learned the skills of a Master Assassin. However his temper and arrogance were still present, which led to clashes with the leadership of the British Brotherhood.
In 1714, his Mentor asked him to sail to the West Indies to rendezvous with the Brotherhood there and meet with their Mentor Ah Tabai. Duncan eagerly accepted the mission, and the British Brotherhood never heard from him again.