Dearden attended all four sessions of the Second Vatican Council between 1962 and 1965. He played an influential role at the Council, helping develop key documents like "Lumen gentium" and "Gaudium et spes". During the Council, he became more progressive in his views, becoming known as a "favorite of the liberals in the church." He dedicated himself to implementing the reforms of the Council, promoting the greater participation of the laity in diocesan affairs, encouraging the formation of a priests' senate, and ordaining married deacons. When Pope Paul VI agreed with the idea raised by participants in Vatican II and restored the permanent diaconate in 1967, Dearden was the first in America to utilize the pronouncement and augmented the declining numbers of regular clergy in his diocese by ordaining thirteen married laymen as deacons in 1971.