Roman Catholic Bishop Thomas C. Kelly, 50, general secretary of the National Conference of Catholic Bishops and auxiliary bishop of Washington, has been named archbishop of Louisville.

He succeeds Archbishop Thomas J. McDonough, 70, who resigned for health reasons last September.

The appointment, announced yesterday by the papal representative in this country, Archbishop Pio Laghi, follows a recent pattern in the church of appointing promising young leaders to geographical areas where Catholicism has historically not been dominant. The designation of Kelly, a Dominican, also furthers a recent worldwide trend in the church to name members of religious orders to key dioceses.

Kelly has served for nearly five years as general secretary of the hierarchy and has been given generally high marks for his diplomatic and progressive leadership style. Roman Catholic dioceses are autonomous, answerable directly to the Vatican, but the bishops collaborate nationally through the NCCB and its sister organization, the United States Catholic Conference, which Kelly also serves as general secretary.

The general secretary for the two bodies administers the programs agreed on by the bishops in their annual assembly and is frequently called on to represent the church's social and political concerns before the legislative and executive branches of the federal government.

In commenting on the appointment, Archbishop John R. Roach of St. Paul and Minneapolis, president of the bishops' conference, praised Kelly for carrying out his tasks "with unfailing sensitivity and compassion for persons. I know of few men who have combined the roles of administrator and pastor so well and so gracefully."

Archbishop James A. Hickey of Washington offered "best wishes and congratulations" to Kelly on the appointment. Although the general secretary job is a full-time post, Kelly preaches and celebrates mass regularly in Washington-area churches.

The general secretary's office, with its high visibility in the nation's capital and its leadership role for the entire hierarchy, has emerged in recent years as a unique seasoning post. Kelly's immediate predecessor, Bishop James A. Rausch of Phoenix, had assumed leadership on a number of issues, particularly in the realm of social justice, and appeared headed for an even wider role in the church, when he died unexpectedly of a heart attack earlier this year.

Kelly served for six years as associate general secretary of the NCCB before being named to his present post in February 1977. Before going to the NCCB he worked for six years as secretary and archivist at the Apostolic Delegation here, the liaison office of the Vatican with the church in this country.

A native of Rochester, N.Y., Kelly earned his theology degree from the Dominican House of Studies here and a doctorate in canon law from the University of St. Thomas in Rome.

He will be installed in his new post in mid-February. His successor is expected to be named by the administrative committee of the NCCB at its regular meeting in March.