His career as a seaman really began in 1777, and two years later he obtained command of a ship. On 23 April 1794, as Commodore of the frigate squadron off the north-west French coast assisting in the blockade of Brest, Warren and his squadron captured a number of French frigates. In 1795, he commanded one of the two squadrons carrying troops for the Quiberon expedition and in 1796 his frigate squadron off Brest is said to have captured or destroyed 220 vessels. In October 1798, a French fleet—carrying 5,000 men—sailed from Brest intending to invade Ireland. The plan was frustrated in no small part due to the squadron under his command during the action of 12 October 1798 off Donegal.