Upon the death of Bishop Daniel J. Feeney in 1969, Peter L. Gerety became the Diocese of Portland's eighth Bishop. He had actually had the title of Apostolic Administrator two years prior, with the authority to carry on the day-to-day activities of the Diocese during former Bishop Feeney's illness. Bishop Feeney had actively sought out an Auxiliary Bishop to his Diocese before his illness. As a result, on March 4, 1966, Pope Paul VI appointed Peter L. Gerety to serve in Portland. His consecration took place in Hartford, Connecticut, June 1, 1966.

Bishop Gerety was a native of Shelton, Connecticut, had attended St. Thomas Seminary in Bloomfield and St. Sulpice Seminary in France. His ordination took place on June 29, 1939, at the Cathedral of Notre Dame in Paris. He served in several parishes in New Haven and later became the first pastor of St. Martin de Porres.

While in Connecticut, Bishop Gerety was an energetic activist for civil rights and an advocate for programs to alleviate poverty. He was chosen, in 1963, as coordinator and director of the Diocesan Priests' Conference on Interracial Justice. During the Bishop's tenure in the Diocese of Portland, he continued his efforts for the cause of social justice by providing suitable housing for the elderly and expanding the scope of the Diocesan Bureau of Human Relations.

At the time Gerety was appointed Auxiliary Bishop for the Diocese of Portland, funds had been allocated for the renovation of the Cathedral in keeping with the changes of Vatican II. Because there was no clear consensus as to what exactly was to be done and because of Bishop Feeney's illness, the role of mediator and decision-maker fell to Bishop Gerety. In order to have more active participation by the faithful (one of the objectives of Vatican II), he undertook to implement what he perceived would be necessary physical alterations in the Cathedral's interior to accomplish this goal. Among his more controversial changes were the removal of the exquisite marble high altar, the Bishop's throne, the pulpit and the communion rail, as well as the addition of plain wooden rectangular panels suspended between the Sanctuary's columns. Needless to say, following this 1969 renovation, the magnificent neo-Gothic Cathedral had a more modern appearance.

In 1974 Bishop Gerety left Maine when he was appointed Archbishop of Newark, New Jersey, the sixth largest Diocese in the United States at the time.