[4] After unsuccessfully coordinating US opposition to Juan Perón in the 1946 Argentine election, he directed US diplomats in Latin America to avoid supporting particular candidates in elections—lest they suffer due to the perceived association.

Everybody likes to take a swing at us, and makes sure he does every time you say something.Mann sought military assistance from Latin American countries during the Korean War, commenting "that if the Bolivians were complaining about spilling their blood for Yankees, a lot of Yankees were also complaining about American blood already being spilled in Korea for Bolivia and other countries of the hemisphere".

[6] In a surprise to many observers, he agreed to secure US aid for Bolivia following the 1952 Bolivian revolution, partly as a reward for the new government's agreement to compensate US tin companies for nationalized assets.

The memo argued that the main issue for the US in this region was not a Communist invasion, but the problem of US control over "readily accessible essential strategic materials".

Mann advocated swift US intervention to retaliate against nationalizations, as a show of force to deter similar actions by other countries.

Although he resisted early overtures by United Fruit representatives to intervene, he opposed Árbenz's land reform law, fearing that Guatemala would provide a test case for other nations.

[11] Mann advocated policies of robust economic assistance, establishing the Inter-American Development Bank and promoting low-interest loans financed by the US government.

[12] President John F. Kennedy promoted the Alliance for Progress, a centrist initiative to support Latin American economies and stave off communism through moderate reform.

[16] Kennedy appointed Mann United States Ambassador to Mexico where he successfully negotiated a settlement of the Chamizal border between the US and Mexican governments, caused by a shift in the Rio Grande.

On December 21, Johnson also made Mann the head of the US Agency for International Development (USAID), an organization created by President Kennedy two years earlier.

[24] After the coup, Mann stated that "the frustration of Communistic objectives in Brazil was the single most important victory for freedom in the hemisphere in recent years".

[25] In Chile, Mann ordered an intensive and coordinated campaign in favor of Eduardo Frei against Salvador Allende in the 1964 elections.

In a May 1 memo to Secretary of State Dean Rusk, Mann wrote:[26] Clearly, the September election will be determined by factors which are deeply rooted in the political, economic, and social fabric of the Chilean scene and by the campaign abilities of the major contenders.

Given the consequences, however, if this major Latin American nation should become the first country in the hemisphere to freely choose an avowed Marxist as its elected president, the Department, CIA, and other agencies have embarked on a major campaign to prevent Allende's election and to support Frei, the only candidate who has a chance of beating him.Mann described a ten-point plan, which included:[27] These efforts were successful in 1964 but reversed in 1970.

[30] In the Dominican Republic, Mann labeled democratically elected President Juan Bosch a communist and supported the US invasion in 1965.

In April 1965, Mann personally insisted on the production of a cable which would describe danger to American citizens in the Dominican Republic.