Durnford's duties changed with the outbreak of the American Revolutionary War. Spanish forces, allied with France and the American colonial rebels, moved to capture British West Florida. January 1780 found Durnford in command of Fort Charlotte in Mobile, an old French fort in disrepair that had been formerly known as Fort Conde. His force of 287 regulars and irregulars was opposed by a Spanish force of 2,000 troops, that arrived in Mobile Bay on 10 February 1780. Given that Durnford was greatly outnumbered the Spanish demanded the surrender of the British force, but Durnford refused, hoping for a relief force from Pensacola. The opposing commanders carried on protracted and polite negotiations over wine and cigars, while the Spanish prepared to take the old fort. Relief was not forthcoming and on 10 March 1780 Durnford surrendered, his estate having been burnt.