The Rev. Thomas Cajetan Kelly yesterday was consecrated a bishop of the Roman Catholic Church by the two men who preceded him as top officers of the American Catholic heirarchy and a third with whom he will serve as auxiliary bishop of the Washington Archdiocese.

In a solemn 2 1/2-hour rite at the Shrine of the Immaculate Conception the 47-year-old Dominican became a prelate of the Church in a centuries-old ceremony marked by joyous overtones and the presence of more than 70 other bishops, including five American cardinals.

Bishop Kelly answered with a vigorous "I am" to a series of questions put to him by Archbishop Joseph Bernardin of Cincinnati on his readiness to accept the duties and responsibilities of the episcopal office.

Then, after a litany invoking the interception of the saints of the Church the slim, white-robed Dominican knelt. Archbishop Bernardin stepped forward and, in silence, laid his hands on the new bishop's head.

As Archbishop Bernardin returned to his seat, Bishop James S. Rausch of Phoenix, under whom Bishop Kelly had served as assistant general secretary of the National Conference of Catholic Bishops, laid his hands on Bishop Kelly's head, and Washington Auxiliary Bishop Eugene A. Marino, the other co-consecrator, followed suit.

Bishop Kelly continued to kneel, head bowed, as the gathering of bishops and cardinals filed past in silence to "impose hands" on the newest of their number.

It is through this "imposition of hands" from a bishop already consecrated to a new prelate-designate that the "apostolic succession" of the leadership of the Church is maintained in an unbroken line back to the first apostles of Christ.

Bishop Kelly will devote full time to his responsibilities at the National Conference of Catholic Bishops and its service organization, the United States Catholic Conference. He became general secretary of these two agencies in March.

His assignment as auxiliary bishop of Washington is primarily a matter of convenience since the Church does not designate at-large bishops. However he will assist with confirmations and do some preaching here. He said in an interview last week that he welcomes such parish duties "to keep me in touch with the reality that is the Church."

Bishop Kelly is the only Dominican currently serving as a bishop in the United States, and is one of only four or five members of any religious order to serve in the American hierarchy.

The Dominican Order gave the new bishop the pectoral cross of an earlier Dominica prelate, Bishop Edward Fenwick, who in 1821 became the first bishop of Cincinnati.

During the eucharistic service, of which the ordination was a part, the newly consecrated bishop left the altar during the kiss of peace in the mass to greet his mother seated in a front pew.

The several thousand persons who crowded into the Shrine for the ceremony burst into applause as the smiling Bishop Kelly walked down the center aisle, giving his blessing during the recessional.