Lawford served in the Boer War in South Africa as a captain in the British Army. His second marriage was on 20 May 1914 in London to Muriel Williams. During World War I, Lawford was promoted to the rank of major-general, and commanded the 41st Division, the junior division of the New Army, throughout its existence. His nickname of 'Swanky Syd', used by others behind his back, apparently derives from his habit of donning full dress regalia for every occasion, including all medals. He was knighted in the field.
Lawford was promoted to lieutenant-general, the third highest rank in the British Army, and was posted to India after World War I. While serving in India in the early 1920s, and while still married to Muriel, he fell in love with the wife of one of his officers, May Somerville Aylen, and she became pregnant with his child. Colonel Ernest Aylen, May's husband, upon hearing this news, divorced her over the scandal. (He died on 12 October 1947).
General Lawford and Muriel divorced. He then married May Aylen, and their child was Peter Lawford (born in 1923). The Lawfords returned to England. But the scandal eventually drove the family to settle in France, and they then moved to the United States by the late 1930s. Peter had appeared in his first film before age ten, learned several languages, and also appeared in several movies in France.