Mr. Clark moved from the No. 2 job in the State Department to the national security post after his predecessor, Richard Allen, resigned in January 1982. When Reagan announced his appointment, he promised the job would be upgraded with a “direct reporting relationship to the president.” The Times said Mr. Clark used this access to become the administration’s “most influential foreign policy official.”

Newsweek said he had returned the role of security adviser to the “lofty status” it had under Henry A. Kissinger in the Nixon administration.

After Interior Secretary James G. Watt was forced to resign after a joke that simultaneously managed to offend blacks, women and the disabled, Mr. Clark volunteered to take the job. Starting in November 1983, he was credited with polishing the agency’s tattered image. He stayed in the post for the rest of Reagan’s first term.

William Patrick Clark Jr. was born in Oxnard, Calif., on Oct. 23, 1931, a fifth-generation Californian. His paternal grandfather, Robert Emmett Clark, was a sheriff who put down a cattle war with his Colt .45 revolver, known in the old West as the Peacemaker. Mr. Clark kept the gun in his White House office as a symbol of American strength. Reagan named the MX nuclear missile the Peacekeeper after this gun.

Mr. Clark’s father, William Pettit Clark, was a cattleman and police chief in Oxnard. In a 1993 interview with Edmund Morris for the book “Dutch: A Memoir of Ronald Reagan,” Mr. Clark compared his relationship with his father to the one he had with Reagan.

The younger Mr. Clark said his father never thanked him for any favor. “Didn’t have to,” he said in his laconic manner. “I knew he was grateful. Ronald Reagan was the same. Whenever he came into the room, as governor or president, he didn’t need to say anything. I could tell what he wanted. Just like when my dad brought stock into the corral: I never had to ask him which gate I should open.”

William Clark, who attended Roman Catholic schools, went to Stanford, but left to study for the priesthood at the Augustine Novitiate in New Hamburg, N.Y. Only two hours of speech were permitted each day. After a year, he briefly returned to Stanford, then moved on to the University of Santa Clara, but did not graduate.