Papadopoulos was born in Elaiohori, a small village in the Prefecture of Achaea in Peloponnese to local schoolteacher Christos Papadopoulos and his wife Chrysoula. He was the eldest son and had two brothers, Konstantinos and Haralambos. After finishing high school in 1937 he enrolled in the Hellenic Military Academy, completing its three-year program in 1940.

During World War II. Papadopoulos saw field action as an artillery second lieutenant against both Italian and Nazi German forces which attacked Greece on 6 April 1941. During the subsequent occupation of Greece by Nazi Germany, Italy and Bulgaria, he worked in the Greek administration’s “Patras Food Supply Office” under the command of Colonel Kourkoulakos, who was responsible for the formation of the “Security Battalions” in Patras which “hunted down” Greek resistance fighters. These were collaborationist military units created by the Greek puppet government of Ioannis Rallis in 1943 to support the German occupation troops. They were supported by the extreme right and pro-Nazi elements, but also by some centrist politicians who were concerned about the dominance of ELAS (the military arm of the communist-dominated National Liberation Front EAM) as the leading group in the Greek resistance. Among the members of the Security Battalions one could find ex-army officers, violently conscripted soldiers, ultra-right fanatics and social outcasts, as well as common opportunists who believed the Axis would win the war.

Papadopoulos married his first wife, Niki Vasileiadi, in 1941. They had two children, a son and a daughter. The marriage, however, ran into difficulty later and they eventually separated. The separation, however lengthy, could not lead to divorce at first because, under Greece’s restrictive divorce laws of that era, spousal consent was required. To remedy this, in 1970, as Prime Minister of the dictatorship he decreed a custom-made divorce law with a strict time limit (and a built-in sunset clause) that enabled him to get the divorce. After having served its purpose, the law eventually expired automatically. After the divorce, Papadopoulos married his long-time paramour Despina Gaspari in 1970, with whom he had a daughter.

He was promoted to captain in 1946; and in 1949, during the Greek Civil War, to major. (See also Greek military ranks.) He served in the KYP Intelligence Service from 1959 to 1964 as the main contact between the KYP and the top CIA operative in Greece, John Fatseas, after training at the CIA in 1953.

Papadopoulos was also a member of the court-martial in the first trial of the well-known Greek communist leader Nikos Beloyannis, in 1951. At that trial, Beloyannis was sentenced to death for the crime of being a member of the Communist Party, which was banned at that time in Greece following the Greek Civil War. The death sentence pronounced after this trial was not carried out, but Beloyannis was put on trial again in early 1952, this time for alleged espionage, following the discovery of radio transmitters used by undercover Greek communists to communicate with the exiled leadership of the Party in the Soviet Union. At the end of this trial, he was sentenced to death and immediately taken out and shot. Papadopoulos was not involved in this second trial. The Beloyannis trials were highly controversial in Greece, and many Greeks consider that, like many Greek communists at the time, Beloyannis was shot for his political beliefs, rather than any real crimes. The trial was by court-martial under Greek anti-insurgency legislation enacted at the time of the Greek Civil War which remained in force even though the war had ended.

In 1956, Papadopoulos took part in a failed coup attempt against King Paul of Greece. In 1958, he helped create the Office of Military Studies, a surveillance authority, under General Gogousis. It was from this same office that the subsequently successful coup of 21 April 1967 emanated.

In 1964, Papadopoulos was transferred to an artillery division in Thrace by decree of Center Union Defense Minister Garoufalias. In June 1965, days before the onset of the major political turmoil known as Apostasia, he made national headlines after arresting two soldiers under his command and eight leftist civilians from settlements near his military camp, on charges that they had conspired to sabotage army vehicles by pouring sugar into the vehicles’ gas tanks. The ten were imprisoned and tortured, but it was eventually proven that Papadopoulos himself had sabotaged the vehicles. Andreas Papandreou wrote in his memoirs that Papadopoulos wanted to prove that under the Center Union government, the communists had been left free to undermine national security. Even after this scandal, Papadopoulos was not discharged from the army since prime minister Georgios Papandreou forgave him as a compatriot of his father. In 1967, Papadopoulos was promoted to colonel.

A failed assassination attempt against Papadopoulos was perpetrated by Alexandros Panagoulis in the morning of 13 August 1968, when Papadopoulos was driven from his summer residence in Lagonisi to Athens, escorted by his personal security motorcycles and cars. Panagoulis ignited a bomb at a point of the coastal road where the limousine carrying Papadopoulos would have to slow down, but the bomb failed to harm Papadopoulos. Panagoulis was captured a few hours later in a nearby sea cave, since the boat sent to help him escape was instructed to leave at a specific time and he couldn’t swim there on time due to strong sea currents. After his arrest, he was taken to the Greek Military Police (EAT-ESA) offices where he was questioned, beaten and tortured. On 17 November 1968, Panagoulis was sentenced to death but was personally pardoned by Papadopoulos, served only five years in prison, and after democracy was restored was elected a member of Parliament. He was regarded as an emblematic figure of the struggle to restore democracy, and as such has often been paralleled to Harmodius and Aristogeiton, two ancient Athenians known for their assassination tyrannicide of Hipparchus.

Constantine appointed a new government nominally headed by Konstantinos Kollias. However, from the early stages, Papadopoulos was the strongman of the new regime. He was appointed Minister of National Defense and Minister of the Presidency in the Kollias government, and his position was further enhanced after the king’s abortive counter-coup on 13 December, when he replaced Kollias as Prime Minister. Not content with that, on 21 March 1972, he nominated himself Regent of Greece, succeeding Georgios Zoitakis.

As internal dissatisfaction grew in the early 1970s, and especially after an abortive coup by the Navy in early 1973, Papadopoulos attempted to legitimize the regime by beginning a gradual “democratization” (see also the article on Metapolitefsi). On 1 June 1973, he abolished the monarchy and declared Greece a republic with himself as president. He was confirmed in office via a controversial referendum. He furthermore sought the support of the old political establishment, but secured only the cooperation of Spiros Markezinis, who became Prime Minister. Concurrently, many restrictions were lifted and the army’s role significantly reduced. An interim constitution created a presidential republic, which vested sweeping—almost dictatorial—powers in the hands of the president. The decision to return to (at least nominal) civilian rule and the restriction of the army’s role was resented by many of the regime’s supporters, whose dissatisfaction with Papadopoulos would become evident a few months later.

On 1 July 1973, The Observer published an article by Charles Foley claiming that the Central Intelligence Agency had engineered the coup and that unnamed senior officials in the Joint United States Military Aid Assistance Group in Athens regarded Papadopoulos as “the first CIA agent to become Premier of a European country”. The source for much of Foley’s story was Andreas Papandreou, the Minister of State in Charge of Intelligence in the government overthrown by Papadopoulos. The following day during William Colby’s confirmation hearings to be Director of Central Intelligence, Colby was asked by Stuart Symington, chairman of the United States Senate Committee on Armed Services, if there was any justification for the assertions. Colby replied that he had the allegations researched and found that the CIA had not engineered the coup, Papadopoulos was not an agent of the CIA, and that Papadopoulos was never paid by the CIA. Colby added “[Papadopoulos] has been an official of the Greek Government at various times, and in those periods from time to time we worked with him in his official capacity.” A clarifying statement was added to the record: “The only association the Agency ever had with Papadopoulos of any kind was in his capacity as an officer of the Greek Intelligence Service, with which we have maintained a liaison relationship since the Greek civil war in the late 1940s.”

After the events of the student uprising of 17 November at the National Technical University of Athens (see Athens Polytechnic uprising), the dictatorship was overthrown on 25 November 1973 by hardline elements in the Army. The outcry over Papadopoulos’s extensive reliance on the army to quell the student uprising gave Brigadier Dimitrios Ioannidis a pretext to oust him and replace him as the new strongman of the regime. Papadopoulos was put under house arrest at his villa, while Greece returned to an “orthodox” military dictatorship.

After democracy was restored in 1974, during the period of metapolitefsi (“regime change”), Papadopoulos and his cohorts were tried for high treason, mutiny, torture, and other crimes and misdemeanors.

On 23 August 1975, he and several others were found guilty and were sentenced to death, which was later commuted to life imprisonment. Papadopoulos remained in prison, rejecting an amnesty offer that required that he acknowledge his past record and express remorse, until his death on 27 June 1999 at age 80 in a hospital in Athens, where he had been treated for cancer since 1996.

John M. Maury, who was the CIA’s Chief of Station in Athens, stated in 1977 that “considerable speculation arose throughout Athens and in the American embassy about the possibility that the Greek military, basically rightist and pro-NATO, might intervene to thwart the election or, if the Center Union party won, prevent the Papandreous from assuming power” and that “some embassy staffers suggested the possibility of a covert CIA operation to encourage the candidacy of moderate pro-Western elements to strengthen the anti-Papandreou forces at the polls”. Maury stated that “a modest covert program to support moderate candidates in a few ‘swing’ districts” was considered by the United States National Security Council, but rejected for fear of irreparably damaging Greece–United States relations and because “the time had come for the Greeks to take care of themselves”. According to Maury, Operation Prometheus caught everyone, including the Americans, by surprise. Maury added that he “had met some of [the brigadiers and colonels left in control after the coup], including George Papadopoulos, who was to head the junta, casually when they were middle-grade officers in KYP, the intelligence service with which CIA had working-level liaison on matters of common concern, as with the intelligence services of all NATO countries.” He described them as “right-wing fanatics” who had no “close connection with the Americans or experience in foreign policy or political activity.”

His biographical notes, published as a booklet by supporters in 1980, mention that he took a civil engineering course at the Polytechneion but did not graduate.