Dempsey, serving as a platoon commander in D Company, first saw action during the Battle of Delville Wood in late July 1916, part of the larger Somme offensive. The battalion, although successful in its role, had suffered heavy casualties, including eight officers, and was relieved in the line and saw little further fighting throughout the year. Dempsey was promoted to acting captain and assumed command of D Company, and later B Company. In November the battalion took part in an assault on Munich Trench, near the River Serre. As at Delville Wood earlier in the year, the assault was successful but with heavy losses, although Dempsey again remained unscathed, and soon returned to England for home leave. On 8 February 1917 he became the adjutant of the battalion.