Armstrong died at Cragside on 27 December 1900, aged 90, and was buried beside his wife in the churchyard at Rothbury. His gravestone carries an epitaph: "His scientific attainments gained him a world wide celebrity and his great philanthropy the gratitude of the poor". Cragside, and Armstrong's fortune, were inherited by his great-nephew, William Watson-Armstrong. Watson-Armstrong lacked Armstrong's commercial acumen and a series of poor financial investments led to the sale of much of the great art collection in 1910. In 1972, the death of Watson-Armstrong's heir, William John Montagu Watson-Armstrong, saw the house and estate threatened by large-scale residential development, intended to raise the money to pay a large inheritance tax bill. In 1971, when advising the National Trust on the most important Victorian houses to be preserved for the nation in the event of their sale, Mark Girouard had identified Cragside as the top priority. A major campaign saw the house and grounds acquired by the Trust in 1977, with the aid of a grant from the National Land Fund.