William P. Clark died at 81. (Photo by James K. W. Atherton / The Washington Post)

William P. Clark, a close adviser to Ronald Reagan who served the president as national security adviser and interior secretary, and also by riding his horse, a white Lipizzaner stallion named Amadeus, died Aug. 10 at his ranch in Shandon, Calif. He was 81.

The cause was Parkinson’s disease, said his son Paul Clark.

Mr. Clark’s association with Reagan dated to the future president’s successful gubernatorial race in California in 1966, when Mr. Clark impressed him as a county-level campaign organizer. For the next two decades, Mr. Clark’s career followed Reagan’s to the height of influence in the White House. It was sometimes said that Reagan regarded Mr. Clark as a brother.

As California governor, Reagan made Mr. Clark his chief of staff and, later, a justice of the state supreme court. When he was elected president, Reagan brought Mr. Clark to Washington, initially as deputy secretary of state under Alexander M. Haig Jr. in 1981. By the time Mr. Clark returned to his ranch in 1985, he was widely known as one of the most powerful figures of Reagan’s first term.

His clout derived not from expertise or longevity but from the personal affinity and basic values he shared with the president. The two men believed fervently in the virtues of small government, military might and anti-Communist resolve — and in the transcendent pleasure of horseback riding.

Before assuming his State Department post, Mr. Clark endured criticism for his performance in Senate confirmation hearings in which he admitted that he did not know such information as the names of key African heads of state. When he first reported to Haig, the Army general was said to have told him: “You, Bill, are going to run the building. I’m going to run the world.”

But Mr. Clark — a rancher who passed the California bar examination on his second try and without graduating from college or law school — earned the esteem of Haig and other officials who valued his loyalty to the president and his tireless efforts to learn what he did not know.

And with his cowboy boots and Stetson, Mr. Clark exuded a degree of authenticity not found in all Washington insiders. He woke before dawn to study the issues of the day — and also to ride Amadeus down the Mall.

At the State Department, Mr. Clark served essentially as Reagan’s handpicked representative and as an intermediary between the temperamental Haig and White House leaders. In early 1982, Mr. Clark replaced Richard V. Allen as national security adviser, a position in which he gave the president daily briefings on foreign affairs.

Mr. Clark was said to have been particularly influential in the field of arms control and was described as a principal player in advancing Reagan’s policy of “peace through strength.” He was a strong proponent of increasing defense spending, exerting economic pressure on the Soviet Union and beefing up U.S. military involvement in Central America. But he reportedly frustrated some Capitol Hill leaders and administration players, including first lady Nancy Reagan, by failing to act without what they considered sufficient collaboration.

Amid turf wars and infighting, Mr. Clark replaced the deeply unpopular James G. Watt as interior secretary in 1983.

He stayed in the position for less than two years. In 1985, when Mr. Clark returned to his law practice in California, The Washington Post credited him with restoring “the department to its job as steward of vast reaches of this country’s land and water.”

William Patrick Clark Jr. was born Oct. 23, 1931, in Oxnard, Calif. The grandson of a county sheriff and the son of a police chief, he grew up on ranches and attended rural schools.

He attended Stanford University but did not fit in socially. After studying to become a Catholic priest, he returned to Stanford, where a dean reportedly suggested that he go back to farming or enroll in another school. Mr. Clark studied at Loyola Law School in Los Angeles before serving in the 1950s in the Army Counter Intelligence Corps in Europe, where he met a German refu­gee who became his wife.

Joan Brauner Clark died in 2009 after more than 50 years of marriage. Survivors include five children, Monica Clark Copeland of Half Moon Bay, Calif., Pete Clark and Paul Clark, both of Shandon, Nina Negranti of San Luis Obispo, Calif., and Colin Clark of Larkspur, Colo.; two sisters; nine grandchildren; and a great-grandson.

It was said that the bond between Mr. Clark and Reagan was so close that they prayed together.

“They had a code word, the ‘DP’ — the Divine Plan. It was insider language,” Paul Kengor, Mr. Clark’s co-biographer, told the Washington Times. “They’d go out riding horses, and Clark would say, ‘Part of the DP,’ and Reagan would say, ‘Yes, Bill, part of the DP.’ ”