The collection consists principally of the duke's correspondence, both his private papers and copies of his official letters, while in public office. The papers include letters and reports arising from his position as Chief Commissioner for Woods 1841-1846, notably concerning various building works in London and at Windsor Castle. The extensive correspondence and related papers as Chief Secretary for Ireland in 1846 reflect all aspects of the growing crisis caused by the failure of the potato crop and the government's responses to it.
The largest sections are devoted to his controversial period as Secretary for War during the Crimean War 1853-1856 and also as Colonial Secretary during the years 1852-1864. All parts of the then British colonial empire are featured in the papers but of special significance are sections relating to Australia, New Zealand, Van Dieman's Land [Tasmania], Southern Africa and British North America - including the Prince of Wales' [later Edward VII's] tour of the U.S.A. and Canada in 1860.
Particularly noteworthy correspondents include Queen Victoria, Prince Albert, Lord Raglan, Lord Elgin, Lord Palmerston, William Gladstone, Sydney Herbert, Sir Randolph Routh and Sir Charles Trevelyan.
There are smaller sections of personal, family and estate papers which touch on his strained relationship with his father, the 4th Duke of Newcastle under Lyne, with his wife, Lady Susan Hamilton - from whom he was divorced in 1850 - and with his children.
Due to the size of this collection the sections are delivered separately on the Manuscripts Online Catalogue:
Ne C 8779-12842, Political and official correspondence of Henry Pelham-Clinton, 5th Duke of Newcastle under Lyne
Ne C 12843-14145, Personal and estate correspondence of Henry Pelham-Clinton, 5th Duke of Newcastle under Lyne