The British Prime Minister during World War I was David Lloyd George. He assumed office in December 1916 and served until October 1922. Lloyd George played a vital role in coordinating the British war effort and was instrumental in various military strategies and negotiations with allies. His leadership was crucial during a challenging and transformative period for Britain, marked by significant military losses and societal changes. He also championed various domestic reforms post-war, which laid the groundwork for the modern welfare state in Britain. His tenure is often remembered for its focus on both wartime leadership and subsequent social change.