This series contains two personal letters, 1833-1834, and wastebooks probably kept by Frothingham's grandfather Richard Frothingham (1748-1819), 1783-1800. This Richard Frothingham was a coach maker in Charlestown and also served as an officer in the Continental Army during the Revolutionary War. He married Mary Kettell in 1778, and their firstborn son Richard (b. 1781) was also a coach maker. The wastebooks contain charges for leather, seats, harnesses, spokes, springs, and chaise bodies. Also included in this series is an unidentified household expense book, 1783-1795.

This series consists primarily of research notes and original documents collected by Frothingham for his History of Charlestown, Massachusetts (Boston: 1845). Among the original documents are lists of those infected or inoculated for smallpox, 1730; documents pertaining to the non-importation of tea, 1773; lists of prisoners taken and accounts of losses sustained during the Battle of Bunker Hill; and a 1776 account of losses sustained when Charlestown was burned by the British during the battle. Charlestown town documents include town meeting minutes, votes, petitions, taxes, and letters to selectmen.

This series also contains Revolutionary-era materials apart from Charlestown, including records of the Cambridge Committee of Correspondence, 1776-1779; three manuscript notebooks (a list of officers, a list of men on board, and an account of provisions) concerning the ship Intrepid in Boston, 1782; and two diaries kept by American soldiers during the Siege of Boston. One is a manuscript copy of part of the diary kept by Samuel Bixby, a private from Sutton in Learned's Regiment, in Roxbury from 4 May 1775-3 January 1776. The other is a diary kept by John Kettell (probably a relation of Frothingham's through his grandmother, Mary Kettell Frothingham) from 17 May-1 October 1775 in Cambridge, Prospect Hill, and Roxbury.

This series, the largest in the collection, consists of correspondence, returns, accounts, stock certificates, and other records of the Middlesex and other canals, including the Amoskeag, Blodget, Bow, Hooksett, and Union canals. The records of the other canals, mostly financial, probably came to Frothingham as a result of their connection with the Middlesex Canal, of which Frothingham served as an officer and treasurer. Most of the records relate to land transactions and the passing of traffic through the various canals and locks.

The Proprietors of the Middlesex Canal received a charter in 1793, and work was begun in 1794. The canal company was formed as part of a nationwide movement in the 1790s to open inaccessible timber and farm lands for transportation and trade. In the case of the Middlesex Canal, the canal increased Boston's trade by making lands in northern Massachusetts and New Hampshire accessible. The Middlesex Canal was a model of civil engineering and served as a prototype for other inland waterways. The company was financed through the sales of shares, which sold for $25 in 1794 and $473 in 1803 upon completion of the canal, although the company did not pay its first dividend until 1819.

This series consists of petitions, bonds, articles of agreement, and other records relating to land disputes in Georgia. As a result of the loose land policies in Georgia in the late 18th century, many fraudulent land grants were acquired and resold. (The Georgia Company was one of four "Yazoo" fraud companies involved in the selling of Georgia land. There were many lawsuits over the Yazoo claims, the final one being Fletcher v. Peck in 1810.) The connection between the Georgia Company and R. Frothingham is unclear.