Back on the ranch in San Luis Obispo County, Calif., he is just plain Bill, a gangling, unassuming neighbor who likes to ride and swap jokes with fellow ranchers.

On the California Supreme Court he has a reputation as a strick law-and-order conservative who frequently writes the dissents to the liberal opinions of Chief Justice Rose Bird.

But in the inner circles of those who have been with Ronald Reagan since Day 1, William P. Clark will be remember best as the chief of staff who came aboard in 1967 when the governor's office was in a shambles and made it run smoothly.

Reagan thought so much of what Clark did that he promoted him to the state high court in defiance of the merit system he used with many of his judicial appointments. When questioned about it, Reagan said he knew Clark's work so well that there was no need to have checked with bar association sources.

Now Clark is coming to Washington at the behest of the man he once brought to Sacramento as Reagan's extradition and clemency secretary, White House counselor Edwin Meese III. When Clark left the Reagan administration in 1970 for a series of judgeships that culminated in his supreme court appointment, he was replaced by Meese, who has been Reagan's closest staff aide since.

Wherever he has gone, Clark has been controversial at first but has wound up making friends in unexpected quarters. One reason is his sense of humor, which surfaced recently when he said that his foreign policy experience had been limited to "72 hours in Santiago."

But Clark's administrative talents are well-demonstrated, and he expects to get a lot of help in the State Department.

"I'm surrounded by expertise in every area," he said yesterday. "I expect to take full advantage of that experise."

Clark also said he expects to have "a good relationship" with Secretary of State Alexander M. Haig Jr., and took issue with press reports that Haig had tried to block his nomination.

In Sacramento, Clark established reasonably good working relationships with the Democratic-controlled legislature at a time when Reagan was far more hostile to the legislative branch than he is now. And Clark was consistently accessible to the press, a practice he usually maintained on the court.

Clark, 49, was named to a superior court judgeship by Reagan and then to the court of appeals, largely uncontroversial appointments. But in 1973, when Reagan appointed Clark to the supreme court bench, reporters discovered that he had never completed either undergraduate studies or law school, where records show he was "disqualified on account of deficient scholarship."

The state bar, after an investigation, found nothing to disqualify Clark from serving on the court. His opinions, though highly conservative, have rarely been faulted on intellectual grounds. And those opinions may one day be of great interest to legal scholars, since there are those in the Reagan entourage who expect to see Clark on the U.S. Supreme Court some day.

When he was named to the California high court, Clark took the usual step of keeping his residence on his 900-acre grain and cattle ranch more than 200 miles from Sacramento. He flew his own airplane to work, but his German-born wife, Joan, vetoed this form of commuting after Clark crashed during a landing at the ranch.

After that, Clark frequently made the trip by bus, spending the week in San Francisco and seeing his wife and their five children on weekends.

Clark is a soft-spoken Reagan loyalist with a strong patriotic streak. In coming to Washington, he said yesterday, he is "totally committed to the presidenths policy of peace through strength."

The deputy secretary of state-designate also is considered something of a workaholic. He spent his 1969 vacation in Washington, helping Caspar W. Weinberger, now secretary of defense, reorganize the Federal Trade Commission.

Clark will take a substantial pay cut in his new job. He received $82,000 as a California supreme court justice, and will make $60,662 in his new post.

While the expectation within the Reagan administration is that Clark will be confirmed quickly by the Senate, he has kept his supreme court post pending the hearings.