Dempsey was commissioned in the British Army in 1915 and fought in France during World War I. He was a lieutenant colonel when World War II began in 1939, and he commanded an infantry brigade in France that helped to cover the British rear guard during the evacuation from Dunkirk (World War II) in May–June 1940. Promoted to lieutenant general, Dempsey in November 1942 took command of the XIII Corps of the Eighth Army in North Africa under General Bernard Montgomery (Montgomery, Bernard Law Montgomery, 1st Viscount). Dempsey's corps formed the right wing of Montgomery's forces in the invasion of Sicily in July 1943, and that September it spearheaded the invasion of the “toe” of the Italian peninsula across the Strait of Messina. Dempsey led the XIII Corps 300 miles (480 km) northward along Italy's west coast in 17 days to link up with Lieutenant General Mark Clark (Clark, Mark)'s American forces at Salerno.

Montgomery picked the quietly competent and methodical Dempsey to command the Second Army, which contained several Canadian and Polish units as well as British forces, in the invasion of Normandy (Normandy Invasion) in June 1944. The Second Army landed successfully on Gold (Gold Beach), Juno (Juno Beach), and Sword (Sword Beach) beaches on June 6, drove inland to capture Caen on July 9, and then kept up pressure on the bulk of the German armoured forces while the U.S. First Army to the west broke out of Normandy on July 25. Dempsey led the Second Army in the Battles of Mortain and Falaise and then swept eastward across northern France and Belgium. After taking part in the failed British attempt to capture Arnhem in The Netherlands (September 1944), Dempsey led the Second Army across the Rhine River in late March 1945 and drove northeastward into Germany, capturing Bremen, Hamburg, and Kiel and reaching the Danish frontier by May 1945.

With Germany's surrender, Dempsey served successively as commander in chief of Allied land forces in Southeast Asia (1945–46) and the Middle East (1946–47). He was knighted in 1944 and retired in 1947.