On June 29, 1939, Gerety was ordained a priest for the Diocese of Hartford at the Cathedral of Notre Dame in Paris. Following his return to Connecticut, he was assigned as a curate at St. John the Evangelist Church in New Haven. In addition to his duties at St. John's, he served as a chaplain at Grace-New Haven Hospital. In 1942, he was appointed a curate at St. Brendan Church and director of Blessed Martin de Porres Center, both in New Haven. The center was an interracial social and religious organization that ministered to the African American Catholic community. In 1956, the center became St. Martin de Porres Parish and Gerety was named its first pastor. During his tenure at St. Martin's, Gerety became an outspoken advocate for the Civil Rights Movement and supported programs to eliminate poverty. In 1963, he was chosen as coordinator and director of the Diocesan Priests' Conference on Interracial Justice.
Following the retirement of Archbishop Thomas Aloysius Boland, Gerety was appointed the third Archbishop of Newark, New Jersey, on April 2, 1974. His installation took place at the Cathedral of the Sacred Heart on June 28 of that year. He was the first Archbishop of Newark to actually live in the city of Newark; his predecessors lived in the Llewellyn Park section of West Orange. During his 12-year tenure in Newark, Gerety created the Office of Pastoral Renewal and began a ministry to divorced Catholics. The Office of Pastoral Renewal evolved into RENEW International, an organization now based in Plainfield, New Jersey, that provides faith-sharing resources for small Christian Communities in the United States, Canada, Latin America, and South Africa. Gerety established the Archbishop's Annual Appeal in 1975, and supported the charismatic and ecumenical movements. He also established the Archbishop Gerety Fund for Ecclesiastical History to advance studies in ecclesiastical history, especially the history of Catholicism in the United States. In 1976, Gerety sent a letter to the Democratic National Convention to protest the party's platform on abortion, which he described as "the bloody horror of the callous elimination of hundreds of thousands of God's most defenseless little ones, our own flesh and blood." That same year, he testified before the Senate Foreign Relations Committee on "U.S. Foreign Policy: A Critique from Catholic Tradition," and in 1977 he represented the United States Catholic Conference before the House Ways and Means Committee to offer his view on President Jimmy Carter's "Better Jobs and Income" proposal.
Later life
Gerety resigned as Archbishop of Newark on June 3, 1986. At that time, he declared, "It is well known that a bishop must resign at the age of 75. I will be 74 years old next month and I told the Holy Father in my letter of resignation that for the good of God's Church and for my own peace of mind, I believe it is time of a younger man to take over the reins of office here in Newark. I have done my best and I am very happy now to step aside." He was succeeded by Bishop Theodore Edgar McCarrick, then serving as Bishop of Metuchen.
Death
He died in Totowa, New Jersey, on September 20, 2016, aged 104, and was the world's oldest living Catholic bishop at the time.