This subseries consists primarily of incoming correspondence to Theodore Lyman III from Civil War veterans with whom he served. Among the correspondents are Charles Francis Adams II, Simon Forrester Barstow, Andrew Atkinson Humphreys, George Gordon Meade, and Gouverneur Kemble Warren. Also included is a small amount of correspondence from Charles Francis Adams, John Quincy Adams II, Edward Everett, and John Murray Forbes. Three letters from Edward Everett (October-December 1863) concern his Gettysburg Address. Some 1867 correspondence discusses the failed Petersburg Mine Assault of 1864 and General Ambrose Burnside's role in the disaster.

This subseries includes material related to Theodore Lyman, Jr.'s brief literary career after his graduation from Harvard in 1810; his tenure as the mayor of Boston, 1833-1835; and his philanthropic efforts on behalf of the state reform school at Westborough, Massachusetts, and the Boston Asylum & Farm School for Indigent Boys. Also included is correspondence with Edward Everett and Robert C. Winthrop; an 1817 European journal, kept by Lyman, containing descriptions of meetings with Goethe, Kotzebue, and other German literary figures; notes on revolutions in France and other countries in 1848; and letters and documents relating to the attack on the Ursuline Convent in Charlestown, Massachusetts, in 1834.

This subseries also contains correspondence of George Robert Russell describing his travels and trading activities in Guangzhou (Canton), Manila, and other foreign capitals in the 1830s; business correspondence, receipts, and legal documents pertaining to the affairs of Theodore Lyman, Jr. and his wife's family; and Lyman's notes for an updated version of his The Diplomacy of the United States (1828). Volumes 1-13 are primarily account and expense books related to the estates of Theodore Lyman and Theodore Lyman, Jr., as well as lists of the younger Lyman's investments.

This subseries consists of correspondence, diaries, and other papers of Theodore Lyman III. Correspondence between 1852-1863 documents Lyman's Harvard years, 1851-1855; his postgraduate work with naturalist Louis Agassiz; the beginnings of the Museum of Comparative Zoology at Harvard; Lyman's zoological study of the ophiuran, a type of starfish; his participation in a scientific research expedition off the coast of Florida in 1856; and his tour of the scientific capitals of Europe, 1861-1863. Primary correspondents during this period are Lyman's Harvard classmates Edward I. Browne, Joseph C. Heywood, and Samuel Johnston; his cousin Charles W. Eliot; and his sister Cora Lyman Shaw and her husband Gardiner Howland Shaw. Letters from Cora Lyman Shaw, Gardiner Howland Shaw, and Anna Shaw Curtis, 1861-1863, discuss business, the Civil War, national politics, and the Boston and New York social scenes.

Correspondence between 1863-1865 includes letters from Lyman to his wife, Elizabeth "Mimi" Russell Lyman, from the headquarters of the Army of the Potomac, about military life, the strategic movements of the army, and engagements with Confederate forces at Spotsylvania, the Wilderness, Cold Harbor, and Petersburg. Lyman also describes Benjamin F. Butler, George Armstrong Custer, Ulysses S. Grant, Abraham Lincoln, and Edwin M. Stanton, as well as Robert E. Lee as he appeared at the Appomattox surrender. Correspondence after 1865 deals with Lyman's post-war life and career, including family matters, his service on the Harvard Board of Overseers, the Harvard presidency of his cousin Charles W. Eliot, Lyman's work with the Massachusetts Commission of Inland Fisheries after 1866, his election to Congress as an independent civil service reformer in 1882, and his painful physical decline after 1886. Included are typescripts of Eliot-Lyman correspondence and Lyman diary entries about Eliot at Harvard.

The subseries also contains correspondence, newspaper clippings, and official documents concerning the mob attack on abolitionist William Lloyd Garrison in Boston in 1835 and the conduct of Mayor Theodore Lyman, Jr. during the incident. These materials were compiled by Theodore Lyman III for his pamphlet Papers Relating to the Garrison Mob (1870), a response to a critical lecture by Wendell Phillips in 1869. Also included are newspaper clippings, speeches, notes, lists, and printed material on a variety of subjects related to Theodore Lyman III, arranged in alphabetical files.

Volumes in this subseries consist primarily of 31 diaries kept by Lyman, 1863-1886. Vols. 14-26 contain detailed, almost daily accounts of his Civil War experiences and his post-war career in science and politics. Vols. 27-44 are line-a-day diaries with brief entries of appointments, meetings, and activities. Additional volumes include a small book of manuscript maps, a notebook of the Lyman estate, scrapbooks of Lyman's political career, and letterbooks containing handwritten copies of his Civil War letters to his wife, 1863-1865.