He was born at Birr Castle, Parsonstown, King's County, Ireland, the son and heir of the astronomer William Parsons, 3rd Earl of Rosse who built the "Leviathan of Parsonstown" telescope, largest of its day, and his wife, the Countess of Rosse (née Mary Field), an amateur astronomer and pioneering photographer. Lawrence succeeded his father in 1867 and was educated first at home by tutors, like John Purser, and after at Trinity College, Dublin and Oxford University. He was the brother of Charles Algernon Parsons, inventor of the steam turbine.