Following the outbreak of the Second World War Barrowclough volunteered for service with the Second New Zealand Expeditionary Force (2NZEF). He commanded an infantry brigade in the 2nd New Zealand Division during the campaign in Greece and Operation Crusader. In 1942 he was recommended for divisional command by his superior officer, Major General Bernard Freyberg, and was given the 3rd Division, which he led through the Solomon Islands campaign. When the division was disbanded in late 1944, he was left without a command and was eventually discharged from the 2NZEF. He returned to the legal profession and in 1953 was appointed Chief Justice of New Zealand. He established a permanent Court of Appeal for New Zealand and also served on the Privy Council for a time. He retired in 1966 and died in Auckland in 1972 at the age of 77.