Slocock had a seemingly premature international retirement, but this was due to his career in the cotton trade, requiring frequent trips abroad, most commonly to the United States. As amateurs, business took priority over sport. He married in 1912 Lena (Elinor) Cook, and they had one son, Anthony, born in 1914. The family moved to Savannah, Georgia, United States. However, Lancelot returned to England to fight in the First World War. He was commissioned as a Second Lieutenant in the 1/10th Battalion of the King's Regiment (Liverpool), a Territorial Battalion, more commonly known as Liverpool Scottish. He arrived in France in January 1916, joining his battalion, the 1/10th, as part of the 55th division. He was then to take part in the Somme offensive, and was involved in the push to capture the village of Guillemont. The first attacks in this area on 8 August had failed and were followed by further attacks on the 9th, two of them made by the 1/10th Battalion of the King’s Regiment. Second Lieutenant Lancelot Slocock whilst leading his men in one of these attacks was killed in action. Another former international, Lance Corporal John Abbott King, also serving with the 1/10th Battalion, died in the offensive. They were two of the eighty two thousand casualties incurred in this part of the campaign, resulting in a total advance of one thousand yards.