Mary Mcgrory President Reagan's new national security affairs adviser, William P. Clark, is a fifth-generation Californian. He has black hair, round brown eyes and rosy cheeks. With his boyish and benign face, he looks like a Norman Rockwell version of a judge.

All, with the possible exception of Rose Elizabeth Bird, who served with him on the California Supreme Court, attest to his niceness. Chief Justice Bird suspects Clark--wrongly, he insists--of bringing about hearings on her fitness for office.

His gifts as a conciliator are widely praised, although not by the British--who bristled at his mention, on a recent trip to Dublin, of U.S. hopes for the reunification of Ireland.

The former deputy secretary of state has imposed an embargo on interviews for the next two or three months, while he breaks into his new White House job. He has received an extraordinary press, based primarily on his feat of having housebroken the wayward secretary of state, Alexander M. Haig Jr., who, under Clark's tutelage, has learned to behave like a team player.

Getting along with Alexander Haig is not the equivalent of getting along with the rest of the world, but Clark's fans trust that his ability to calm the volatile will translate into some positive benefit at a moment of supreme confusion and embarrassment about America's world role.

Another reason that the Clark appointment was received with such enthusiasm is because of what he is not, as much as what he is. He is not a careerist. His heart is in his 900-acre ranch in the California county where his grandfather, Robert Emmett Clark, served as sheriff. His pistol and star are enshrined in his grandson's State Department office.

More importantly, Clark is no academic. In fact, he dropped out of Stanford and Loyola Law School. He displayed his encyclopedic lack of information about the world at his Senate confirmation hearings.

In view of recent history--in view, that is, of a string of NSC warrior-professors who dressed up dubious policies in scholarly trappings--Clark's innocence of Ivy League connections is a positive recommendation.

It was under McGeorge Bundy of Harvard, it is hopefully recalled now, that President Kennedy took the first fateful steps into Vietnam; Walt W. Rostow of the Massachusetts Institute of Technology quickened the march into the quagmire; Henry A. Kissinger of Harvard authored the most brutal bombing in history; Zbigniew Brzezinski of Columbia posed at the Afghan border with a machine gun.

Small wonder then that someone who doesn't have to prove his machismo in the faculty lounge, and whose background is in state politics, is receiving such a welcome.

Clark's relationship with Ronald Reagan is easy and warm. Clark's mild manner commends him to a president who appreciates the laid-back approach. Reagan's confidence in his one-time California chief of staff is so complete that he gave him a mandate to deal with the thorny problems of South Africa and Northern Ireland outside of the bureaucracy.

Ideologically, they are twins. Like Reagan, Clark sees the malevolent hand of the Soviets in every problem area from Belfast to El Salvador. Like Reagan, he regards conflict in terms of good versus evil.

Those who know him best say Clark's abiding motivation is religious rather than ideological. He is a fervent, conservative Catholic in the "traditional" mold, who refers to Pope Leo XIII rather than John XXIII, and who prefers the Latin Mass.

He may be one of those whom Washington's Archbishop James Hickey had in mind when, in introducing Archbishop Obando y Bravo of Nicaragua, he spoke of "many people who do not understand the modern church . . . , who are limited to a view that predates the Second Vatican Council . . . , who do not take into account the church's preferential position for the poor."

Clark is one of those high-level Catholics at the State Department--Haig is another --who are baffled and chagrined by the opposition of the Catholic hierarchy to White House policy on El Salvador, by the adamant refusal, especially by Hickey, to see the civil war in anticommunist terms. At their November conference, the bishops turned a deaf ear to State's clerical Salvadoran ally, Bishop Pedro Aparicio-- "treated him like a disease," Clark was heard to complain.

But Clark, some hope, will balance the tilt against Europe prevalent among westerners in national security circles. Defense Secretary Caspar W. Weinberger, for instance, seems to see the continent as a depot for nuclear arms and nerve gas. Clark is married to a Czechoslovakian, Joan Brauner, whom he met while an enlisted man in Army counterintelligence in West Germany, and he reveres the Old World.

In all, much good will attends the successor of the forgetful Richard V. Allen as national security adviser. No change in direction is anticipated, but Clark might be able to arrange a change in tone, which would be progress of sorts.