Berryman joined the Second Australian Imperial Force (AIF) on 4 April 1940 with the rank of full colonel, and became General Staff Officer Grade 1 (GSO1) of the 6th Division. He was responsible for the staff work for the attacks on Bardia and Tobruk. In January 1941, Berryman became Commander, Royal Artillery, 7th Division, and was promoted to brigadier. During the Syria-Lebanon campaign, he commanded "Berryforce". He returned to Australia in 1942, becoming Major General, General Staff, of the First Army. Later that year, he became Deputy Chief of the General Staff under the Commander in Chief, General Sir Thomas Blamey, who brought him up to Port Moresby to simultaneously act as chief of staff of New Guinea Force. Berryman was intimately involved with the planning and execution of the Salamaua–Lae campaign and the Huon Peninsula campaign. In November 1943 he became acting commander of II Corps, which he led in the Battle of Sio. In the final part of the war, he was Blamey's representative at General of the Army Douglas MacArthur's headquarters and the Australian Army representative at the Japanese surrender in Tokyo Bay.