Darby was appointed to the Board of Admiralty as First Naval Lord in the North ministry in September 1780. In April 1781 he relieved Gibraltar from its siege by the Spanish, for the second time during that war. This event is recorded in a full-length portrait by George Romney, painted 1783–6, which hangs in the National Maritime Museum. On the change of ministry in April 1782 he resigned his command and did not again serve at sea; he also resigned his seat on the Admiralty Board. He was Member of Parliament for Plymouth, from 1780 to 1784. He lived at Newtown House at Newtown in Hampshire and died in 1790.