Upon the mobilisation of the TF in August 1914, shortly after the
British entry into World War I, Lawford was promoted to the temporary rank of
brigadier general on 5 August and continued to command his brigade. On 7 September, he became the first
general officer commanding (GOC) of the newly organised
22nd Infantry Brigade, which formed part of the
7th Division under Major General
Thompson Capper. Lawford led the brigade through the division's initial engagements while serving with the
British Expeditionary Force (BEF) on the
Western Front, most notably the
First Battle of Ypres, "during which he personally led attacks, sometimes on horseback, armed only with a cane". In February 1915, he was appointed a
Companion of the Order of the Bath (CB) "for services rendered in connection with Operations in the Field". Throughout the early months of 1915, he led his brigade through the battles of
Neuve Chapelle,
Aubers Ridge, and
Festubert. During 1915, Lawford twice served as acting commander of the 7th Division during the absence of Capper. He first served as acting GOC from 6 to 19 April 1915, shortly before the commencement of the
Second Battle of Ypres. He handed over divisional command to Major General
Hubert Gough on 19 April, returning to lead the 22nd Brigade during the ensuing battle. He resumed acting command of the 7th Division for a second time, this time lasting from 14 to 19 July, until Capper's return from convalescence. In September 1915, Lawford was promoted to temporary major general and appointed GOC of the newly formed
41st Division, the most junior division of the
New Armies created by
Field Marshal Lord Kitchener. Somewhat unusually, he would serve as its GOC for the rest of the conflict, ending the war "as the longest serving divisional commander in the British Expeditionary Force". He oversaw the division's initial training in England and its 1916 deployment to France, leading them during the
Battle of the Somme, specifically at
Flers–Courcelette. He was promoted to the substantive rank of major general on 1 January 1917, "for distinguished service in the field". He continued to lead his division throughout 1917. He led his division through the
Battle of Messines and the
Battle of Passchendaele before being redeployed to the
Italian Front in November, remaining there until March 1918 during which he was knighted in the field when his CB was upgraded to a
Knight Commander of the Order of the Bath (KCB) in January 1918 "for valuable services rendered in connection with Military Operations in the Field". After returning to France, he led the division through the
German spring offensive and the final
Hundred Days Offensive, which ultimately led to the
Armistice of 11 November 1918 and the end of the war. His military nickname was "Swanky Syd", apparently derived from his habit of donning full dress regalia, including all of his medal entitlement, regularly. General
Sir Douglas Haig, then commanding the
First Army of the BEF, noted in his personal diary in early 1915 the following assessment of Lawford, then still commanding the 22nd Brigade, as a commander: ==Postwar and final years==