Sir Nicholas became a high ranking officer and distinguished soldier under King Edward I. His early career is obscure, but, in 1278, he received Royal letters of protection upon travelling to Ireland for two years on the King's service. In 1299, he gave letters of attorney, as he was again going abroad with Aymer de Valence, the Earl of Pembroke. By the following year, he was serving in the King's army in Scotland where he was present at the Siege of Caerlaverock Castle in the July. One can imagine that he fought bravely in many of King Edward's Scottish, Welsh and French campaigns. His reputation was such that he attended the Parliament called at Lincoln, in January 1301, to consider the letter from Pope Boniface VIII claiming his position as feudal overlord of Scotland. He signed the barons reply as 'Lord of Moulsford', but because he had not actually received a writ of summons, his position as Lord Carew is not generally accepted. Later, in June that same year, he was summoned, as from Devon, to perform military service in person against the Scots and to attend the muster at Berwick. In August 1305, he was appointed Constable of Bothwell Castle.