Guillemard's 80 or so letters to his family from America include correspondence primarily with this uncles, James and Isaac Guillemard, merchants in London. There are also some letters addressed to his sister, Jeanne Marie Griffin, and a few to Guillemard's nieces Frances, Jane, and Mary Griffin. There are also a couple of letters to and from Guillemard's brother-in-law, Davies Giddy. The letters are written principally from Philadelphia (Pa.), including "The Solitude," John Penn (grandson of William Penn) and his estate located on the outskirts of the city; the French émigré colony of Asylum, (Pa.); New York; Boston; Quebec; and Montreal.

These 14 letters written from Paris, Nancy, Strasbourg, Besançon, Geneva, Zurich and elsewhere and addressed to Guillemard's sister Jeanne Marie Griffin and his father Jean Guillemard, document two of Guillemard's journeys-- the first in 1787-1788 and the second in 1789-- to France and Switzerland prior to and at the outset of the French Revolution. Also included are three typed copies of Guillemard's letters written circa August and September 1789 to Newton Ogle, aide-de-camp to Earl Grey, with whom he witnessed the aftermath of the capture of the Bastille. There is also an account Guillemard wrote in 1823 detailing his experiences in France with Ogle, which includes documentation of an attack that occurred while they were staying in Roeun, and their witnessing in front of the Palais Royale the procession transporting the heads of Jacques de Flesselles and Bernard René de Launay.

Serving as her teacher when she was a child, John Guillemard was particularly close to his niece Jane Griffin Franklin (1792-1875), the second wife of naval officer, Arctic explorer, and lieutenant-governor of Van Diemen's Land (now Tasmania) Sir John Franklin (1786-1847).

Writing from London, Clavering, Rome, Naples, Liancourt (France) and elsewhere, Guillemard's 11 letters to Sir George Thomas Staunton (1781-1859) in Canton (Guangzhou) primarily relate to the latest scientific discoveries, such as Humphry Davy's electrical battery; news about the Royal Society and the Royal Institution; his financial affairs as well as Staunton's; and the political and economic situations in England, France, and Italy. Writing from Italy during the Neapolitan War, some of Guillemard's letters include accounts of his experiences. While visiting Mount Vesuvius, Guillemard writes that he is, "surrounded...by dust and smoke and flame and the thunder of artillery and the uproar of a people mad with novelty and joy...(May 23, 1815)"

Consists of about 50 letters to Guillemard's nieces, Frances Griffin (d. 1868), Jane Griffin Franklin (1791–1875), and Mary Griffin (1793–1854), from London, Tredrea (the Cornish home of Guillemard's brother-in-law Davies Giddy), Clavering, Paris, Florence, Rome, Naples and elsewhere.

Consists of about 20 letters from Guillemard mainly to his father, John Guillemard, sister, Jeanne Marie Griffin, cousins, and uncles, James and Isaac Guillemard. Also included are a couple of letters written by his father, John Guillemard, to family members.

Includes a description of Guillemard's detainment in quarantine in the Lazaretto of Leghorn.