 Born to a Royal Navy warrant officer of Irish extraction, also named John Conn, he was baptised at Stoke Damerel, Devon, on 5 August 1764.Conn first went to sea in 1778, aged thirteen, aboard on his father's ship , before securing a place on as a midshipman, and in which he saw action at the battle of the Saintes in April 1782.In 1788 he was made a lieutenant, but had to wait five years before being given a good position, using the intervening time to marry Margaret Nelson, a vicar's daughter.