The papers of U.S. Army officer and governor of New Jersey George Brinton McClellan (1826-1885) consist of 33,000 items (59,477 images), most of which were digitized from 82 reels of previously produced microfilm. Spanning the years 1783-1898, with the bulk dating from 1850 to 1885, the collection contains correspondence, telegrams, memoranda, diaries, writings, notes, military papers, McClellan family papers, scrapbooks, printed copies of speeches, articles and books, and other papers relating primarily to General George B. McClellan's Civil War service, particularly in the 1862 Peninsular Campaign in Virginia and battle of Antietam in Maryland.

McClellan's first letters in the collection were written during his years as a cadet at the United States Military Academy at West Point, 1842-1846. Later letters are from Mexico, where he was on duty during the Mexican War. Additional information on the war is contained in a diary, 1846-1853. Correspondence and another diary for the period during which he served as an instructor at West Point, 1848-1851, are also in the collection.

A letterbook and several notebooks relate to McClellan's years of service (1851-1854) as a member of an exploring party on the Red River, as engineer in charge of an expedition in Texas, and on engineering duty in Oregon and Washington, where for a time he was construction engineer in the western division of the Northern Pacific Railroad. Incoming letters, a letterbook, reports, and notebooks bear on McClellan's mission to Europe (1855-1856) as a member of the Delafield military commission, a board of officers assigned to study European military systems. McClellan's correspondence from 1857 to 1861 illuminates his successive roles as engineer and executive of privately owned railroad companies.

Over half of the collection, however, concerns the period of McClellan's 1861-1862 service in the field during the Civil War, with particular emphasis on the Peninsular Campaign and the battle of Antietam. Post-Civil War correspondence concerns his 1865-1868 tour abroad, invitations extended to him to accept the presidencies of the University of California and Union College, a project for reorganization of the United States Army, his service as chief engineer of the New York City Department of Docks, his governorship of New Jersey, and his interest in the National Home for Disabled Volunteer Soldiers.

Correspondents represented in the collection include Benjamin Alvord, John Jacob Astor, Nathaniel Prentiss Banks, Samuel L. M. Barlow, J. G. Barnard, Don Carlos Buell, Ambrose Everett Burnside, Simon Cameron, George Ticknor Curtis, William Dennison, John A. Dix, Edward Everett, Millard Fillmore, William Buel Franklin, Joshua R. Giddings, Ulysses S. Grant, H. W. Halleck, Samuel Peter Heintzelman, Rufus Ingalls, Joseph E. Johnston, Amos Kendall, Hiram Ketchum, Robert E. Lee, Abraham Lincoln, Manton Marble, Randolph Barnes Marcy, Irvin McDowell, George Gordon Meade, Louis Philippe Albert d'Orléáns (comte de Paris), Allan Pinkerton. Fitz-John Porter, William Cowper Prime, William S. Rosecrans, Winfield Scott, Edwin McMasters Stanton, Isaac Ingall Stevens, Edwin V. Sumner, E. D. Townsend, Clement L. Vallandigham, Stewart Van Vliet, Daniel W. Voorhees, John Ellis Wool, and members of the McClellan family.

General McClellan was commonly referred to during his lifetime as George B. McClellan, although his authorized Library of Congress name authority of George Brinton McClellan is reflected in the title of his collection of papers. McClellan's son and namesake was also known as George B. McClellan, and a collection of his papers also reside in the Manuscript Division at the Library of Congress as the George B. McClellan Papers. Some authors have cited these collections as the George B. McClellan, Sr. Papers and George B. McClellan, Jr. Papers, respectively.

A finding aid (PDF and HTML) to the George Brinton McClellan Papers is available online with links to the digital content on this site.

The collection is arranged in twelve series:

• Series 1: Correspondence I, 1783-1888 (Reels 1-43)
  Contains correspondence, transcripts of correspondence, and related military papers, business papers, reports and speeches, financial matter, and memorabilia. The material is arranged chronologically.
• Series 2: Correspondence II, 1823-1898 (Reels 43-61)
  Consists of correspondence and related military papers, articles, and printed campaign material. The material is arranged chronologically.
• Series 3: Letterbooks and Telegram Books, 1852-1862 (Reels 61-66)
  Includes letterpress copies, fair copies, drafts, and extracts of outgoing letters and telegrams, as well as two volumes of deciphered telegrams sent and received. Several of the volumes overlap in time and subject matter covered. The material in this series is arranged by the type of material and therein chronologically.
• Series 4: Special Correspondence, 1870-1873 (Reel 66)
  Consists of correspondence of the New York City Department of Docks–Engineers' Bureau, with the Board correspondence indexed in a separate volume. The material is arranged in part in reverse chronological order as originally received by the Library of Congress.
• Series 5: Diaries, 1846-1884 (Reels 66-68)
  Contains diaries kept by George Brinton McClellan; his wife, Mary Ellen ("Nell" or "Nelly") Marcy McClellan; his children, May and George B. ("Max") McClellan; and his brother, Arthur McClellan. The diaries are arranged by name of author and therein chronologically.
• Series 6: Speech, Article, and Book File, 1864-1885 (Reels 68-72)
  Includes manuscripts and printed copies of speeches, articles, and books. The series is arranged by type of material and therein chronologically.
• Series 7: Military Papers, 1846-1862 (Reels 72-75)
  Consists of army orders, receipts, vouchers, inventories and lists, which are arranged chronologically.
• Series 8: Notebooks, 1842-1885 (Reels 75-77)
  Contains engineers' field books, note and memorandum books kept by George Brinton McClellan, Arthur McClellan, Joseph F. Minter, and Robert Johnson. The material is arranged chronologically.
• Series 9: Financial Papers, 1842-1885 (Reels 77-79)
  Includes account books, bills, receipts and vouchers, which are arranged by decade.
• Series 10: Miscellany, 1852-1885 (Reels 79-82)
  Contains memorabilia, campaign literature, passports, photographs, newspaper clippings, materials related to McClellan's death, scrapbooks, diplomas, awards, memorials, certificates, and commissions. The series is arranged by type of material.
• Series 11: Addition, 1866-1872 (Not filmed; scanned from originals)
  Includes three diaries of Mary Ellen Marcy McClellan and three unidentified photographs. Her diaries include personal observations and direct transcriptions of General McClellan's thoughts and views on his meetings with famous world leaders during his European travels. There are also reflections on General McClellan's military and political service during the Civil War. These diaries complement her earlier volumes in the Diaries series.
• Series 12: Oversize
  Contains an oversize scrapbook, and oversize certificates, awards, and citations, which were filmed in their original locations before removal to the Oversize series. The material is arranged and described according to the series, containers, and folders from which the items were removed.