 Educated at The King's School, Sydney, and Brighton Grammar, Melbourne, Pentland went on to study dairy farming at Hawkesbury Agricultural College, and later worked as a jackaroo.His father was a physician who joined the Australian Imperial Force (AIF) during World War I and served as a major in the Australian Army Medical Corps.Pentland enlisted as a private in the AIF on 5 March 1915, sailing for Egypt with the 12th Light Horse Regiment aboard HMAT A29 "Suevic" on 13 June.