Series 1, Correspondence, 1942-1968, is arranged chronologically, and consists primarily of carbon copies of letters Trimble sent, though there are some letters that he received. As he rose in rank, the volume of correspondence increases. Each posting created a different range of correspondents and concerns. In general, Trimble's correspondence is routine in nature. Primarily in English, some correspondence is in the local language of the country in which he served, specifically Spanish, Portuguese, German and French.

The earliest period of correspondence, 1942-1954, is rather sparse. Nothing is included in these papers from Trimble's earliest postings nor from his service during World War II. The only exception is three items in Spanish dating from 1942-1943 while he was posted to Mexico. While posted to Iceland, most of the correspondence is personal in nature, much of it congratulations on promotion. Little has been preserved of his service in London. While at The Hague, most of the correspondence refers to matters such as reductions in embassy staff and Dutch aid. Correspondents include Tyler Thompson, Executive Director of the Bureau of European Affairs, and Livingston T. Merchant, Assistant Secretary of State.

Trimble's correspondence increases with his appointment as Counselor at Rio de Janeiro in 1954. Correspondence between Trimble and the consulates around Brazil detail the information exchange process. Trimble and Sterling Cottrell (Officer in Charge, Brazilian Affairs, Bureau of Inter-American Affairs) regularly corresponded on the political, military, and social aspects of Brazil. This correspondence continued with Edward Rowell, Cottrell's successor. Several memorandas to the Ambassador relating to a variety of topics, such as the Southeast Asia Treaty Organization (SEATO), United States Information Service (USIS) operations, and conversations held with members of the Brazilian government are found. A charged political atmosphere is strongly evident in his correspondence to Washington and elsewhere. Surprisingly, there is little correspondence in reference to Vice President Nixon's attending the inauguration of President Kubitschek, but there is correspondence suggesting the invitation of president-elect Kubitschek to the United States.

In September 1956, Trimble transferred to Bonn, Germany serving under Ambassador James Conant and later Ambassador David Bruce. Correspondence between Trimble and both men is found in the collection. As the second in command, much of the correspondence is routine, detailing problems with staff, budget cuts, and visits by Congressmen and other high ranking governmental officials. Also found is regular correspondence with Jacques Reinstein, Director of the Office of German Affairs in the Department of State, primarily reporting on day-to-day matters, politics, and war criminals. There are again memoranda to the Ambassador on subjects such as incidents involving U.S. forces and German nationals and records of conversations with other members of the diplomatic community, but little correspondence regarding U.S.-Soviet relations in this period.

When Trimble became head of the embassy in Cambodia in 1959, he corresponded with other members of the diplomatic corps and the Cambodian government. Trimble kept a regular correspondence with J. Graham Parsons, Deputy Assistant Secretary of State in Washington, on the state of Cambodia and the needs of the embassy. Another regular correspondent was the American Ambassador to Vietnam, initially Elbridge Durbrow and later Frederick Nolting, Jr. A series of cables to and from Washington and other posts dating from August 1960 to June 1962 are preserved. These cables are often more detailed than the correspondence of the same period. There is little direct correspondence on the Sam Sary "letter" published in 1960 alleging a conspiracy between the United States and an opponent of Prince Sihanouk. There is a large volume of correspondence from late 1960 to early 1961 on the political upheavals in Laos and Prince Sihanouk's concern on the subject. In late 1960 a large section of the Khmer-American Friendship Highway buckled, an issue Trimble raised frequently with Washington, including a detailed overview sent to newly appointed Assistant Secretary for Far Eastern Affairs, Averell Harriman.

From September 1962 to February 1968 Trimble oversaw aspects of the African Desk at the State Department. He was first appointed Director of the Office of West Coast and Malian Affairs overseeing seven countries and eventually responsible for nineteen African nations. Trimble visited Nigeria, Togo, Ghana, and Liberia in December 1962 and Kenya and Tanganyika (now Tazmania) in January 1964 to gain an understanding of these countries. Much of the correspondence is between him and the ambassadors under his supervision. Some of the issues discussed in his correspondence in this period are how to maintain good relations with Ghana, the coup in Togo in January 1963, problems faced by Peace Corps volunteers, and reactions within these countries to major political events, such as the Cuban Missile crisis. In May 1965 Trimble was promoted to Deputy Assistant Secretary of State for African Affairs and placed in charge of the African Bureau. In this role, he became the person responsible for forwarding Dean Rusk's letter notifying the Congolese government that the United States was closing its embassy. Unfortunately, there is little correspondence relating to this important event. Between 1966 and Trimble's retirement in 1968, most of the correspondence is personal in nature with little reference to his work as Deputy Assistant Secretary.