Mitrione joined the Federal Bureau of Investigation (FBI) in 1959. The following year he was assigned to the State Department's International Cooperation Administration. He was then sent to South America to teach "advanced counterinsurgency techniques." His speciality was in teaching the police how to torture political prisoners without killing them.
According to A.J. Langguth of the New York Times, Mitrione was working for the CIA via the International Development's Office of Public Safety (OPS). We know he was in several foreign countries but between 1960 and 1967 he spent a lot of time in Brazil and was involved in trying to undermine the left-wing president João Goulart, who had taken power after President Juscelino Kubitschek resigned from office in 1961.
João Goulart was a wealthy landowner who was opposed to communism. However, he was in favour of the redistribution of wealth in Brazil. As minister of labour he had increased the minimum wage by 100%. Colonel Vernon Walters, the US military attaché in Brazil, described Goulart as “basically a good man with a guilty conscience for being rich.”
The CIA began to make plans for overthrowing Goulart. A psychological warfare program approved by Henry Kissinger, at the request of telecom giant ITT during his chair of the 40 Committee, sent U.S. PSYOPS disinformation teams to spread fabricated rumors concerning Goulart. John McCloy was asked to set up a channel of communication between the CIA and Jack W. Burford, one of the senior executives of the Hanna Mining Company. In February, 1964, McCloy went to Brazil to hold secret negotiations with Goulart. However, Goulart rejected the deal offered by Hanna Mining.
The following month Lyndon B. Johnson gave the go-ahead for the overthrow of João Goulart (Operation Brother Sam). Colonel Vernon Walters arranged for General Castello Branco to lead the coup. A US naval-carrier task force was ordered to station itself off the Brazilian coast. As it happens, the Brazilian generals did not need the help of the task force. Goulart’s forces were unwilling to defend the democratically elected government and he was forced to go into exile. This action ended democracy in Brazil for more than twenty years. According to David Kaiser (American Tragedy) this event marks the change in the foreign policy developed by John F. Kennedy. Once again, Johnson showed that his policy was to support non-democratic but anti-communist, military dictatorships, and that he had fully abandoned Kennedy’s neutralization policy.
Mitrione remained in Brazil to help the new government deal with the supporters of João Goulart. According to Franco Solinas, Mitrione was also in the Dominican Republic after the 1965 US intervention.
In 1967 Mitrione returned to the United States to share his experiences and expertise on "counterguerilla warfare" at the Agency for International Development (AID), in Washington. In 1969, Mitrione moved to Uruguay, again under the AID, to oversee the Office of Public Safety. At this time the Uruguayan government was led by the very unpopular Colorado Party. Richard Nixon and the CIA feared a possible victory during the elections of the Frente Amplio, a left-wing coalition, on the model of the victory of the Unidad Popular government in Chile, led by Salvador Allende.
The OPS had been helping the local police since 1965, providing them with weapons and training. It is claimed that torture had already been practiced since the 1960s, but Dan Mitrione was reportedly the man who made it routine. He is quoted as having said: "The precise pain, in the precise place, in the precise amount, for the desired effect." It has been alleged that he used homeless people for training purposes, who were allegedly executed once they had served their purpose.
On July 31, 1970, the Tupamaros kidnapped Daniel Mitrione and an Agency for International Development associate, Claude L. Fly. Although they released Fry they proceeded to interrogate Mitrione about his past and the intervention of the U.S. government in Latin American affairs. They also demanded the release of 150 political prisoners. The Uruguayan government, with U.S. backing, refused, and Mitrione was later found dead in a car. He had been shot twice in the head but there was no evidence that he had been tortured.
The Secretary of State William P. Rogers and President Nixon's son-in-law David Eisenhower attended Mitrione's funeral. The Uruguayan ambassador, Hector Luisi, promised that the people responsible for Mitrione's death would "reap the wrath of civilized people everywhere".
A few days after the funeral, a senior Uruguayan police officer, Alejandro Otero, told the Jornal do Brasil that Mitrione had been employed to teach the police to use "violent techniques of torture and repression". The US government issued a statement calling this charge "absolutely false" and insisted he was a genuine member of the Agency for International Development.
In 1978 Manuel Hevia Cosculluela, a CIA agent who had worked with Mitrione in Montevideo, published a book about his experiences (Eight Years with the CIA). According to Cosculluela, Mitrione had tortured four beggars to death with electric shocks at a 1970 seminar to demonstrate his techniques for Uruguayan police trainees. Cosculluela reported that Mitrione worked under William Cantrell, a CIA agent. Mitrione told Cosculluela: "Before all else, you must be efficient. You must cause only the damage that is strictly necessary, not a bit more. We must control our tempers in any case. You have to act with the efficiency and cleanness of a surgeon and with the perfection of an artist. This is a war to the death. Those people are my enemy. This is a hard job, and someone has to do it. It's necessary. Since it's my turn, I'm going to do it to perfection.If I were a boxer, I would try to be the world champion. But I'm not. But though I'm not, in this profession, my profession, I'm the best."
James Abourezk, who represented South Dakota in the U.S. Senate, discovered that the Office of Public Safety had been training Latin America police to torture left-wing activists for many years. Abourezk made this information public and in 1974, Congress banned the provision by the U.S. of training or assistance to foreign police and the OPS was closed down.