A specialist on Latin America, he served as United States Ambassador to Cuba from February 1959 until October 1960, the first months of the Castro regime.

[5] He was Vice Consul and Third Secretary in the US embassy in Havana in 1938 and 1939, followed by a year in Washington as Cuban desk officer at the State Department.

He told Secretary of State Cordell Hull that the Movimiento Nacionalista Revolucionario (MNR) embodied the "legitimate and respectable... aspirations of certain sectors of the Bolivian people."

He maintained friendly relations with opposition politicians, angering Colombian dictator General Gustavo Rojas Pinilla,[5] who persuaded the State Department to reassign him.

[7] In January 1957, representing the US at the United Nations General Assembly's Special Political Committee, he supported a Philippine proposal, endorsed by representatives of Peru, Nepal, and other nations, for the UN to modify its confrontational approach in fighting apartheid in South Africa and to shift to tactics that would promote discussion and recognize the problem of racial discrimination in other countries as well.

[7] In January 1959, Eisenhower named Bonsal United States Ambassador to Cuba just days after Fidel Castro came to power.

The New York Times called his appointment "a splendid choice" and described him as "a distinguished career diplomat" with "every qualification that could be asked for the difficult and gratifying task he is taking on.

When Bonsal testified before a closed session of the House Committee on Foreign Relations in May 1959, he explained why the revolution had such widespread popular support: "the corruption and the sadism of many Batista henchmen united most Cubans against the regime."

"[18] In August, he protested to Secretary of State Herter that Cuban-American relations were being poisoned by the fact that the US was allowing several hundred Batista allies to live in the country, which appeared to the Cubans as harboring counter-revolutionaries.

Bonsal lodged a formal protest with Cuban President Osvaldo Dorticós on October 27 and blamed Castro for the deterioration of relations.

[21] In mid-1960, the Cuban government reached an agreement to sell 700,000 tons of sugar to the Soviet Union, leading to a series of escalating actions by the US and Cuba.

He also described the Pentagon Papers as "stolen property" and objected to those who ignored the violation of government secrecy standards because the revelations supported their political judgment.