The papers of James McHenry span the years 1775 to 1862, with the bulk of the items concentrated in the period 1775 to 1816. The collection consists of correspondence and related documents, memoranda of McHenry as secretary of war in the administrations of George Washington and John Adams, miscellaneous personal, financial, and professional material, and a diary kept during the Constitutional Convention of 1787. The collection is organized into the following series: Correspondence , Miscellany , Addition , and Oversize .

Series A and B of the Correspondence series, which together constitute more than half the collection, are composed almost entirely of negative photocopies received by the Library of Congress from McHenry's heirs between 1927 and 1930. The photocopies were organized into bound volumes and include, in addition to the negatives, a few positive prints obtained in subsequent years from various other donors. Series C and D contain original documents. Although each series of correspondence is organized chronologically, their dates overlap.

McHenry's papers for the war period relate both to personal aspects of his life in the American army and to his role as amanuensis. They include a copy of his will, a 1776 letter to Benjamin Rush, McHenry's oath of allegiance to the United States, and letters to Thomas Sim Lee, governor of Maryland, written after Washington had reassigned McHenry as aide to the Marquis de Lafayette. Correspondence for the Revolutionary era is occasionally enriched by McHenry's detailed observations on the war's progress. A copy of a 12 September 1778 McHenry letter from White Plains, New York, to Hugh Williamson describes the Continental Army's current deployment and the prospects of future English military actions. The early years are further enhanced by letters from Nathanael Greene to McHenry. Series B of the Correspondence includes a photostatic copy of a draft of the Declaration of Independence which Thomas Jefferson circulated through Richard Henry Lee.

Toward the end of the Revolution, McHenry returned to Maryland, where his family owned an importing business, and in 1783 was appointed by the state to the national Congress. A staunch Federalist, McHenry retained and cultivated the friendship of his wartime associates. Letters involving Washington and Lafayette appear in the postwar correspondence, as do those of Charles Carroll of Carrollton, Samuel Chase, William Hindman, William Vans Murray, Timothy Pickering, Charles Cotesworth Pinckney, Benjamin Tallmadge, and Uriah Tracy.

For the years 1796 to 1800, which are the best documented in the collection, the papers cover a wide variety of subjects relating to McHenry's position as secretary of war. Included are reports on frigate construction, fortification of ports and harbors, defense of the frontier, and conferences with various American Indian nations, as well as memoranda on the fluctuating state of affairs among the United States, England, France, and Spain. In July 1796 McHenry submitted to Washington a report on the power of the chief executive to remove a diplomatic officer. In April 1797 he wrote a memorandum on the "quantum and kind of defensive force necessary at this juncture."

In the spring of 1798 McHenry and the War Department became involved in implementing a series of bills passed by Congress which put the country on a war footing. The papers contain substantial correspondence on the issues involved, including letters between and among McHenry, Washington, Alexander Hamilton, Charles Cotesworth Pinckney, and James Wilkinson. The subsequent breach in Adams's cabinet between the Hamiltonian faction, of which McHenry was a part on the one hand, and the president and his supporters on the other, is also well treated, including McHenry's detailed recollection of a conversation which took place between him and President John Adams the evening preceding McHenry's resignation as secretary of war, a copy of which he sent to the president.

McHenry's correspondence continued after he left office, though the bulk is much less. A draft of a long letter McHenry wrote on 20 December 1802 to the speaker of the House of Representatives, Nathaniel Macon, was eventually printed and consists of a refutation of Republican charges that McHenry while in the War Department had abused the public trust. This letter is in Correspondence Series C . Other correspondents in the McHenry papers include Philemon Dickinson, Henry Knox, Benjamin Stoddert, and Oliver Wolcott (1760-1833).

Some of the copies of letters to McHenry dated between 1779 and 1812 in Series D transcribed in 1915 when the originals were in the possession of the McHenry family contain errors in dating.

In the Miscellany series are drafts of writings and legislative bills and McHenry's 1787 diary of the Constitutional Convention at Philadelphia. The diary, which when first published in 1906 shed new light on the convention, remains an invaluable source for the study of the federal constitution. McHenry was in attendance for only a few days during the latter part of May 1787 when a brother's illness forced him to leave, but he returned on 4 August and continued his private account of the proceedings until 8 September. The diary is supplemented by letters McHenry wrote from Philadelphia to his wife and by a speech in the Oversize series he delivered in favor of the new plan of government to the Maryland Assembly.

The Addition , appended in 1999, contains two letters. These are the only items in the papers not microfilmed. The first letter, dated 1797 while McHenry was serving as secretary of war, is a response to Colonel David Henley. The second item is McHenry's draft version of George Washington's 1798 letter to President John Adams accepting the office of commander in chief of the army. McHenry wrote the letter while at Mount Vernon, 11-13 July 1798. Both the draft and the revision were later given to Adams. As the secretary of war, McHenry received and retained the draft.