Yass, New South Wales
Born in Yass, New South Wales, Costello was the fifth child of Michael and Mary Costello. His father was a store-keeper and grazier who had come to Australia with his wife from Ireland in 1837. All four of his siblings died en route to Australia, the family later had a daughter named Mary. The family had settled in Yass in 1851 after selling their store and acquired in the area. Costello quickly became a renowned stockman.