The papers of George Brinton McClellan (1826-1885) span the years 1783-1898, with the bulk of the material dated between 1850 and 1885. The collection includes correspondence; military papers, diaries, notes, writings, financial material, printed matter, and items related to McClellan's death. The papers are organized in twelve series: Correspondence I; Correspondence II; Letters and Telegram Books; Special Correspondence; Diaries; Speech, Article, and Book File; Military Papers; Notebooks; Financial Papers; Miscellany; Addition; and Oversize .

Over half of the collection concerns the period of McClellan's service in the field in the Civil War, 1861-1862, with particular emphasis on the peninsular campaign and the battle of Antietam. Notable are the correspondence, military papers, and material bearing on his report on the Army of the Potomac, 1864, and on his autobiography, 1887. Among the correspondents for the Civil War period are Nathaniel Prentiss Banks, J. G. Barnard, Don Carlos Buell, Ambrose Everett Burnside, Simon Cameron, H. W. Halleck, Samuel Peter Heintzelman, Rufus Ingalls, Abraham Lincoln, Irvin McDowell, Randolph Barnes Marcy (McClellan's father-in-law), George Gordon Meade, Edwin McMasters Stanton, Edwin V. Sumner, Stewart Van Vliet, George H. Weeks, and John Ellis Wool.

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 in a diary, 1846-1853. Letters 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, 1851-1852, 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. Among the correspondents for the period are Benjamin Alvord, J. D. Foster, George Gibbs, Joshua R. Giddings, Randolph Barnes Marcy, Charles E. Mix, Persifor Frazer Smith, and Isaac Ingalls Stevens. Incoming letters, a letterbook, reports, and notebooks bear on McClellan's mission to Europe, 1855-1856, as a member of a board of officers assigned to study European military systems. In his published reports he made numerous recommendations for changes in the United States Army.

McClellan's letters and letters received from such correspondents as William Dennison, Winfield Scott, and E. D. Townsend illuminate his successive roles as engineer and executive of privately-owned railroad companies, 1857-1861. His presidential aspirations and interest in national political affairs, 1864-1865, are reflected in letters from John Jacob Astor, Jr., Leslie Combs, John A. Dix, Millard Fillmore, Amos Kendall, Hiram Ketchum, Manton Marble, Joel Parker, Benjamin Rush, Charles E. Swann, Clement L. Vallandigham, Daniel W. Voorhees, James C. Welling, and Henry Benjamin Whipple.

Post-Civil War correspondence concerns his tour abroad, 1865-1868; invitations extended to him to accept the presidency of the University of California and of Union College, respectively; a project for reorganization of the United States Army; his service as chief engineer of the New York City Department of Docks, 1870-1872; his governorship of New Jersey, 1878-1881; and the National Home for Disabled Volunteer Soldiers. Among the correspondents are F. S. Blount, Ambrose Everett Burnside, William Buell Franklin, Ulysses S. Grant, Samuel Peter Heintzelman, Randolph Barnes Marcy, Thomas B. Peddle, Allan Pinkerton, Fitz-John Porter, William Cooper Prime, Nathaniel Bradstreet Shurtleff, Gustavus Woodson Smith, Stephen W. Tichenor, and Augustus Woodbury.

The collection includes diaries and notebooks kept by people other than McClellan. Among them are members of his family and Joseph F. Minter, who compiled a Comanche alphabet at Fort Vancouver in 1853.

Other correspondents in addition to those named above include Samuel L. M. Barlow, Samuel Sullivan Cox, George Ticknor Curtis, Edward Everett, Joseph E. Johnston, Robert E. Lee, Louis Philippe Albert d'Orleans, comte de Paris, and William S. Rosecrans.

An Addition 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. The diaries complement her earlier volumes in the Diaries series.