In Assassin's Creed Mirage, Basim works as a thief before becoming an Assassin.
Basim begins the game as a street thief in ninth-century Baghdad. After a series of events involving the Order of the Ancients, he leaves his old life and travels to the Hidden Ones' stronghold at Alamut, where he trains under Roshan and becomes an initiate.
Mirage focuses on Basim's early development and presents Baghdad as its central setting. Its story takes place before the events of Assassin's Creed Valhalla, where Basim is introduced as a more experienced member of the Hidden Ones.
Basim is not initially a mercenary or sailor. Those roles describe protagonists from other entries, including Kassandra or Alexios in Odyssey and Edward Kenway in Black Flag. His starting occupation is specifically a thief in Baghdad.