Aristocles Spyrou was born in Vasiliko Pogoni of the then Turkish-occupied Ioannina on the ^6th of April 1886. He was the son of the doctor Matthew Spyros and Eleni née Mokorou. From an early age he showed his inclination towards letters. Thus, at the urging of the later Metropolitan Athenagoras Eleftheriou of Paramythia, he was sent in 1903 to study at the great Theological School of Halki. In 1910, he received his degree in theology, became a monk, was ordained a deacon and took the name Athenagoras. Aware of the importance of communication with East and West, Athenagoras showed an early interest in foreign languages and was able to speak Greek, English, Turkish, French, Albanian, Romanian, Spanish, and Russian.

In 1919, the then Archbishop of Athens Meletios Metaxakis hired him as archdeacon and secretary of the Archdiocese. In December 1922, during the revolutionary government of N. Plastiras, at the age of 36 and while he was still a deacon, he was ordained a priest, bishop and immediately elected Metropolitan of Corfu.

In 1930, he was appointed Archbishop of North and South America by the Holy Patriarchal Synod of the Ecumenical Patriarchate. He served in this position until 1948. During his tenure there, he succeeded in uniting communities divided by the ethnic divide, gave special attention to the building of churches and schools, and founded the Greek Orthodox School of Theology of the Holy Cross in Boston. In the United States he was closely associated with then President Franklin Roosevelt.

In October 1948, the Ecumenical Patriarch Maximos V is forced to resign under pressure from the United States, the United Kingdom, Greece and Turkey. The official version was health reasons. It was rumoured, however, that he was bothered by his pro-Russian politics. There were several candidates for his succession. Greece preferred Archbishop Chrysanthos of Athens; Turkey vetoed Metropolitan Joachim of Derka; the Holy Synod of the Patriarchate probably had a preference for Metropolitan Dionysios of Methymnos.

However, after the strong support of the United Kingdom and the United States and the personal involvement of then President Tecton Harry Truman, Athenagoras was finally elected as the ^268th Ecumenical Patriarch of Constantinople and New Rome on November¹, 1948. He was not enthroned, however, until January ²⁶, 1949. He was not enthroned because there was an obstacle. According to the Treaty of Lausanne and Turkish law, the Ecumenical Patriarch must be a Turkish citizen, which was obviously not the case with Athenagoras. Things had reached an impasse when President Harry Truman intervened, who, on his own initiative, transferred Athenagoras to the President’s official aircraft (the first Air Force aircraft named “Sacred Cow”) and sent him to Istanbul. He told Turkey that it should give an immediate solution to the issue. Indeed, and while the aircraft was flying over the Atlantic, on the pretext that Athenagoras had been born in the Ottoman Empire, he was granted Turkish citizenship.

His importance and the scope of his work is of global and historical magnitude. Athenagoras became the leader and the greatest advocate of the ecumenical movement which had begun as early as 1920. The greatest supporter of the World Council of Churches founded a few months before his enthronement. The meeting between Ecumenical Patriarch Athenagoras and Pope Paul VI is considered historic. This meeting took place on Sunday, January ⁵, 1964 at 9:30 p.m. on the Mount of Olives in Jerusalem for the first time since the schism. Also of historical significance was the lifting of the 1054 Anathemas between the two Churches on December ⁷, 1965.

Patriarch Athenagoras I died on July 7, 1972 in the Greek hospital of Valoukli. His body was taken in procession to the Holy Monastery of the Life-Giving Spring of Valoukli, where he was buried near the graves of the previous Ecumenical Patriarchs.