Clark died Saturday at his ranch home in the central California town of Shandon after a long battle with Parkinson's disease, according to his son.
"It's been a hard fight for him, but we do feel he's in a better place and he's not suffering any longer," Paul Clark said Sunday.
The elder Clark began working for Reagan by managing the actor's 1966 gubernatorial campaign in Ventura County, north of Los Angeles. He ascended to various political jobs as Reagan moved from the Golden State to the White House.
Clark worked for Reagan in Sacramento, rising to the position of executive secretary, before accepting a judgeship with the San Luis Obispo County Superior Court. Reagan later appointed him to the state appellate court in Los Angeles, and then the state Supreme Court, before he moved to Washington to serve as deputy secretary of state and national security adviser.
Clark was national security adviser when Reagan maneuvered the Soviet Union toward arms control, and he was a key player in Reagan's philosophy of "peace through strength." Their close relationship led Time magazine to name Clark "the second most powerful man in the White House" in a 1983 cover story.
The New York Times said Clark had more access to Reagan than anyone else.
"They had very similar ideas about what ought to be done," Edwin Meese, who served as counselor to Reagan and then as attorney general, told the San Luis Obispo Tribune in 2009. "And they also knew and understood one another very well, having worked together back in the California days."
Clark then served as interior secretary for nearly two years, replacing unpopular department head James Watt, before returning to his private law practice and business consulting firm.
Born in 1931 in Oxnard to a family of lawmen (his grandfather Robert was Ventura County sheriff and a U.S. marshal; his father, William Sr., was the police chief of Oxnard), Clark served in the Army Counter-Intelligence Corps in Europe in the mid-1950s. He attended both Stanford University and Loyola Law School without earning degrees, but nevertheless passed the bar exam.
"That's been pointed out throughout his career that he finished neither college nor law school, but be that as it may he did just fine," his son Paul said.
After retiring from public life, Clark and his wife Joan designed and built a chapel in Shandon that they donated to the community. A devout Catholic, he also became a strong abortion opponent.
Clark is survived by five children. His wife died four years ago.
Copyright 2013 The Associated Press.