Phillips Cosby joined the Royal Navy in 1747, and was given command of a schooner at the Battle of Louisbourg in 1758. He was present at the capture of Quebec in 1759. Two years later he was promoted to Captain and took command of HMS Hind, followed by HMS Isis, and, in 1766, of HMS Montreal. In 1771 he was appointed Receiver-General of St Kitts. His next command was HMS Centaur and took part in the Battle of Ushant in 1778. A year later he was in command of HMS Robust, taking part in the Siege of Charlestown in 1780, and in the Battle of Cape Henry the following year. He was appointed Commander-in-Chief, Mediterranean Fleet in 1785, and then of Cork in 1790, and finally Port Admiral at Plymouth in 1792. Although the wall tablet indicates that he was Admiral of the Red, he was also Admiral of the Blue (1795) and Admiral of the White (1801).