Starting in 1870, the diocese added frescoes, a marble baptistery, three altars, a chancel railing, a barrel-domed roof, and a bishop's cathedra to the church.

[3] When the church upgrades were finished in 1905, St. Peter's was ready to be consecrated as a cathedral by the apostolic delegate, Archbishop Diomede Falconio.

[2] The diocese renovated Saint Peter's in 1981 to repair structural damage and to modify the sanctuary to meet liturgical changes after the Second Vatican Council.

Contractors added flying buttresses to support the transept walls in 1991, along with a steel substructure for the pillars and choir loft.

With the assistance of the cathedral's rector, Father Joseph Cocucci, the diocese undertook further renovations at Saint Peter's: On November 22, 2007, in a Thanksgiving Day service at Saint Peter's, Bishop Michael Saltarelli thanked Father Cocucci "for putting the Blessed Sacrament in His proper place and for putting the bishop in his proper place."