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.

Guillemard's detailed and insightful letters document a number of topics, in particular political happenings in the country as well as America's foreign relations with France and England following the Revolutionary War. Guillemard himself becomes involved in the latter when he is asked in May 1797 to serve as the fifth Commissioner to the Jay Treaty to deal with British property claims in America following the war. (While serving, Guillemard makes sure that his uncles do not forward any claims to him personally so as to avoid any hint of bias.) In his last letter dated May 6, 1800, Guillemard discusses America's current political state: "The administration here is in great confusion. The President has removed the Secretary of State, and the Secretary of War has resigned, and the Attorney General executes these offices. It is pretty clear that Mr. Adams does not even expect to be re-elected President, and it is probable Mr. Jefferson will. The measures of the... Men will be not less hostile to England than those of the present party, but they will perhaps have an air of being so, because the hatred to Great Britain which is now hypocritically concealed will then be openly avowed." His reflections on the future of the country are prescient: "This country is much divided in its real and supposed interests. – some of which are founded on geographical situation, some on commercial calculation, some on imaginary or real wrongs. Of these the former are the most likely to break up the Union, but it will not be for many years (May 2, 1797)."

Besides documenting American politics, Guillemard's letters also include observations and information about American society, bankruptcies and financial failures, the activities of land speculators, local insurrections against "oppressors" and tax-gatherers, and outbreaks of yellow fever, which during his stay in Philadelphia, resulted in his taking up residence at "The Solitude". His correspondence also relates to personal matters such as family and financial affairs, including Guillemard's interest in purchasing an estate in England.

Guillemard's letters also include discussions and observations of his travelling companion, the French social reformer François Alexandre Frédéric, duc de La Rochefoucauld-Liancourt. On March 25, 1795, Guillemard writes that he is going to Canada, "with a gentleman who has spent much time in England, whom Fortune once smiled upon...Now that fortune frowns upon him he is still manly and resigned, melancholy at times, but at others enjoying with unfeigned pleasure the good which are left to him." However, de Liancourt was refused entry: "The gate of Paradise was shut upon him", Guillemard supposes in order to "keep him from intercourse with the native Canadians." Guillemard also traveled with de la Rochefoucauld to the small French émigré colony of Asylum in northern Pennsylvania ("On the E. Branch of the Susquehanna, about 180 miles N.W. of Philadelphia"), "an establishment originally imagined by some Frenchmen – with the desire of collecting together the ruined French who have been driven, or have escaped, from France or the French Islands…(May 28, 1795)" (Rochefoucauld documented his American travels in Journal de voyage en Amerique et d'un sejour a Philadelphie, 1 octobre 1794-18 avril 1795.)