Young Jamison excelled as a pupil at his parish school. He married comparatively early, went to live in neighbouring County Antrim, fathered several legitimate children (Mary, John and Jane), and studied to be a surgeon. He decided to join the Royal Navy to advance himself in the world, receiving a naval surgeon's warrant in either 1777 or 1780. In 1786, he was assigned to as a surgeon's mate (apprentice surgeon). "Sirius" had been designated to act as the armed escort to a convoy of convict transports and supply ships that would become known as the First Fleet. The fleet's mission was to establish a permanent British penal colony in New South Wales, on the strategic east coast of Australia.