He was the youngest of the five sons of William Fellowes, physician-extraordinary to the Prince Regent – one of Thomas's brothers was the physician James Fellowes and James's son was the later captain Thomas Fellowes (1827-1923). Serving for a while on ships of the East India Company, Thomas moved to the Royal Navy in 1797 as the master's mate on HMS Royal George. He then moved to HMS Diana and then to other ships before the Peace of Amiens in 1802. When the war broke out again he was deployed to the East Indies under Sir Edward Pellew, rising to lieutenant in 1807 and spending time in the West Indies in HMS Northumberland, Sir Alexander Cochrane's flagship. His first command was the brig HMS Swinger in 1808 as lieutenant-commander, with which he fought at the capture of the island of Dese