Tobias was born near Plymouth and joined the Royal Navy in 1755 and became a midshipman on H.M.S. Marlborough. He was promoted to second lieutenant in 1759 and sailed on the H.M.S Dolphin under Samuel Wallis for the southern hemisphere and returning in 1760.
On 29 November next year he was promoted commander and appointed captain of H.M.S. Adventure under James Cook’s who had overall command