Berryman joined the Second AIF on 4 April 1940 with the rank of full colonel, receiving the AIF serial number of VX20308, and became General Staff Officer Grade 1 (GSO1) of Major General Iven Mackay's 6th Division, in succession to Sydney Rowell who stepped up to become chief of staff of I Corps. Berryman soon established a good working relationship with Mackay. Despite the friction between Militia and Staff Corps officers, Berryman chose to assess officers on performance. This meant that while Berryman viewed some Militia officers, like Brigadier Stanley Savige of the 17th Infantry Brigade, with disdain, he maintained good relations with others. There were also personal and professional rivalries with other Staff Corps officers, such as Alan Vasey. Yet even those who disliked Berryman personally for his lack of patience and tact and referred to him as "Berry the Bastard" respected his abilities as a staff officer.