The collection consists of works, correspondence, documents, photographs, scrapbooks, and miscellaneous material of McClellan reflecting his varied career. There are also many of his letters to his mother, Ellen Marcy McClellan. Works include the autograph manuscript of The Oligarchy of Venice (Boston: Houghton Mifflin, 1904), as well as essays--many about Italy--poetry, speeches (1902-1921), and lecture notes and notebooks for his course on economic history (1912-1930). Documents include wills, memoranda of agreement, and diplomas of the McClellan family (1872-1935), bills to reorganize the army before the House of Representatives (1890-1900), awards to firemen, a veto of a bill to regulate and improve railroad terminals and approaches of the New York Central and Hudson River Railroads in New York City (1893-1917), and honorable discharges and appointments to the regular forces as well as to the Officers Reserve Corps of the U.S. Army (1886-1940). There are photographs and photograph albums, medals and souvenirs of McClellan's political career such as campaign rosettes, and printed material including bound copies of his Spencer Trask (1907) and Stafford Little Lectures (1909), both delivered at Princeton.

The collection also contains some papers of McClellan's father, General George Brinton McClellan (1826-1885), including speeches (1836-1878), correspondence (1861-1888), documents, and miscellaneous material. In addition, there are photographs and photograph albums of McClellan and his wife, army officers, Mathew B. Brady Civil War scenes, and Florence, Italy.