Thomas Jamison possessed a hawk-like visage, a shrewd brain, abundant reserves of energy and a peppery personality. He also had a liking for money, which led to him participate in a series of maritime trading ventures whilst stationed in Sydney. This was in open defiance of government regulations which prohibited public officials from engaging in mercantile enterprises such as the lucrative rum trade. He even acquired the half-share in a merchant ship and formed business partnerships with leading settlers John Macarthur and Garnham Blaxcell.