Born in Masterton, Barrowclough commenced legal studies in 1913 and joined the Territorial Force the same year. In 1915, partway through his law degree, he volunteered for service abroad with the New Zealand Expeditionary Force and fought on the Western Front. He finished the First World War in command of a battalion in the New Zealand Rifle Brigade. After the war he finished his law studies and became a successful lawyer. He also resumed duty in the Territorial Force, serving until 1931 when he moved to Auckland to join a law firm based there.