The exact date of William Ros's birth is unknown. He was described in 1394 as about twenty-three years old, which would place his birth year around 1370. The Ros family was an important one in Lincolnshire and Yorkshire, and the historian Chris Given-Wilson has described them as one of the greatest fourteenth-century baronial families to never receive an earldom. Ros's father was Thomas Ros, 4th Baron Ros, who fought in the Hundred Years War under Edward III (particularly in the Poitiers campaign of 1356). Several years before William's birth, King Edward instructed Thomas Ros to remain with his army on his Irish estates "to prevent the loss and destruction of the country". Thomas married Beatrice, the widow of Maurice Fitzgerald, Earl of Desmond and daughter of the first Earl of Stafford. He died in Uffington, Lincolnshire in June 1384, and his eldest son John—William's elder brother—inherited the title as fifth Baron Ros.