The letters between 1790 and 1823 span King's career as a U.S. Senator, from his first term to his last, and his time spent as Minister to Great Britain. The sustained topic of these letters is the state of King's financial affairs. The letters cast a great deal of light on the investments and financial interests of King and to the manner in which Low controlled them. Along with King's personal finances, the letters often turn to the domestic and international issues of the day. During King's first term as senator, he was especially concerned with the issue of banking in the United States. The early letters in this series include his thoughts on the establishment of the Bank of the United States and the role of the Bank of North America and the Bank of New York. Other topics include the defeat of Arthur St. Clair's army by the Indians, the 1792 campaign for governor in New York, William Duer's financial problems and a plan by Genet to send 2,000 men down the Mississippi River to take New Orleans and Florida.

After assuming the position of Minister to Great Britain in 1796, King's letters move away from the domestic concerns of the United States and are dominated by the war between England and France. From 1796 to 1803, King describes the course of the war, periodic negotiations for peace, developments and shifts in alliances (especially the role of North European powers) and popular opinion towards the war. As a representative from the United States, King's main concerns were the seizure of American ships by England and France, the effects of the war on American trade and the intentions of European powers in North America during and after the war. Of special interest is the letter dated October 10, 1798, which links Thomas Paine with advice given to the French on how to pursue a war against the United States. Along with reports on the war, King discusses trade prospects in Europe, describes domestic affairs in England and Ireland and makes general observations on the political scene in the United States. When King returned to the United States and returned to the Senate, his letters to Low tended to focus much more on his financial affairs and less on the work he was involved with and the issues of the nation.