Carter later called orchestrating the forced exodus of Schlesinger and multiple other members of his Cabinet "a mistake," writing in a diary entry that it raised questions among the public about his leadership abilities.
For his part, Schlesinger said Carter was "just not a natural leader" even though he liked him personally.
"He had a way of discerning things that needed to be done, and yet he was a poor leader in that he did not know the arts of keeping the public with him," Schlesinger said in a 1984 interview with the University of Virginia's Miller Center presidential oral history project.
At the Pentagon, Schlesinger strove to keep the United States from falling behind the Soviet Union in conventional and nuclear forces. He promoted a nuclear strategy that called for precision in hitting military targets without causing huge losses of civilian life and outlined the importance of maintaining forces capable of surviving and responding to nuclear attack.
Pentagon-watchers saw in Schlesinger a rare secretary who emphasized the importance of long-range strategic thinking.
"Incisive, brilliant, thoughtful," said Andrew Krepinevich, executive director for the Center for Strategic and Budgetary Assessments. But he said Schlesinger had "little patience for people who can't keep up with him intellectually."
While at the CIA in 1973, Schlesinger was angered to learn that the spy agency had provided support to ex-CIA agents E. Howard Hunt and James McCord, who were convicted of burglary in the Watergate break-in. He also expressed dismay that the CIA was spying on U.S. citizens in violation of its charter.
He ordered "all senior operating officials of this agency to report to me immediately on any activities ... outside the legislative charter" that barred the CIA from spying inside the United States.
The result was 693 pages of memos about spying on Americans, opening their mail and plotting to kill foreign leaders. Schlesinger's successor, William Colby, shared the contents with the Justice Department and made them available to congressional investigators.
Also, at Schlesinger's direction, a new highway exit sign publicly identifying CIA headquarters for the first time was hung outside its sprawling suburban Langley, Va., campus. Previously, the complex had been disguised — not very convincingly — as a federal highway-research agency.
In his earlier stint at the Atomic Energy Commission, Schlesinger brought his wife and two of his daughters to Amchitka Island in the Aleutians to witness a 1971 nuclear-bomb test and prove to critics that it could be carried out without harm to people or the environment.
James Rodney Schlesinger was born in 1929, in New York City. He earned bachelor's, master's and Ph.D. degrees in economics — all from Harvard.
Schlesinger taught economics at the University of Virginia and published "The Political Economy of National Security," a look at the economics of foreign policy. The Rand Corp. hired him and later he became director of the think tank's strategic studies. In 1969, he joined the Nixon administration in 1969 as assistant White House budget director, specializing in defense matters.
Schlesinger traveled through Western Europe, Africa and Asia in 1950-51 on a fellowship. Some years later, he said, "I learned that the world was a very complicated place and that the narrow discipline of economics gave a narrow insight into the social life of man."
In later years, he served as chairman of the board of the Mitre Corporation, a nonprofit based in Bedford, Mass., and McLean, Va., that operates federally funded defense research and development centers. He also was a longtime member of the Pentagon's Defense Policy Board and served as counselor to the Center for Strategic and International Studies.
He also was a senior adviser to Lehman Brothers until the giant U.S. investment bank collapsed in September 2008, an early casualty of the Great Recession.
In 2008, Defense Secretary Robert Gates picked Schlesinger to lead a task force that made recommendations on improvements in the handling of nuclear weapons. In 2009, he joined six other former CIA directors in asking President Barack Obama to end the Justice Department's criminal probe into the harsh interrogations of terror suspects during the Bush administration.
His wife, born Rachel Mellinger, died of cancer in 1995. She was a concert violinist and board member of the Arlington (Va.) Symphony. They had eight children.
Associated Press writer Matthew Daly contributed to this report.