Weinberger attended San Francisco Polytechnic High School. He was academically gifted and gained admission to Harvard University, graduating in 1938 with a B.A., magna cum laude. Weinberger was offered a scholarship to study at the University of Cambridge but instead attended Harvard Law School, graduating in 1941 with a Bachelor of Laws.^[5]

After graduating from Harvard Law School, Weinberger enlisted in the U.S. Army as a private. He was sent to the Army's Officer Candidate School at Fort Benning, Georgia, where he was commissioned a second lieutenant. During World War II, he served with the 41st Infantry Division in the Pacific; by the war's end, he was a captain on the intelligence staff of General Douglas MacArthur. Early in life, Weinberger developed an interest in politics and history, and, during the war years, a special admiration for Winston Churchill, whom he would later cite as an important influence in his life. From 1945 to 1947, Weinberger was a law clerk for U.S. circuit judge William Edwin Orr of the United States Court of Appeals for the Ninth Circuit. He then entered private practice at a San Francisco law firm.

In 1952, Weinberger entered the race for California's 21st State Assembly district^[6] in the San Francisco Bay area as a Republican at the persuasion of his wife, Jane Weinberger,^[7] who also served as his campaign manager.^[8] He won and was reelected in 1954 and 1956. As the chairman of the Assembly Government Organization Committee, Weinberger was responsible for the creation of the California Department of Water Resources and was instrumental in the creation of the California State Water Project. Weinberger also unsuccessfully opposed the construction of the Embarcadero Freeway, saying it would ruin the view of the Bay and damage property values.^[9] Weinberger felt vindicated when the freeway was removed after the 1989 earthquake. Although unsuccessful in his 1958 campaign for California Attorney General, Weinberger continued to be active in politics and was chosen by Nixon in 1962 to become chairman of the California Republican Party.

Governor Ronald Reagan named him chairman of the Commission on California State Government Organization and Economy in 1967 and appointed him State director of finance early in 1968. Weinberger moved to Washington in January 1970 to become chairman of the Federal Trade Commission. He is credited for having revitalized the FTC by enforcing consumer protection.^[10]

Weinberger subsequently served under President Richard Nixon as deputy director (1970–1972) and director (1972–1973) of the Office of Management and Budget and Secretary of Health, Education, and Welfare (1973–1975). While serving in the Office of Management and Budget, Weinberger earned the nickname "Cap the Knife" for his cost-cutting ability. For the next five years, Weinberger was vice president and general counsel of the Bechtel Corporation in California.

In 1973, the Southern Poverty Law Center named Weinberger as a defendant in a case that sought restitution for the forced non-consensual sterilization and medical experimentation on three young Black American girls, Minnie Lee, Mary Alice, and Katie Relf in Montgomery, Alabama. An employee of Montgomery's federally-funded Community Action organization took the Relf sisters to a family planning clinic under the pretext of needing “shots.” Staff gave Katie Relf a then-experimental birth control shot as well as inserted a contraceptive IUD device without parental knowledge or consent. On a separate occasion, doctors surgically sterilized Minnie Lee and Mary Alice who were twelve and fourteen years old respectively.^[11] At the time of the suit, the Office of Economic Opportunity was preparing to hand over funding and control of its associated family planning clinics to Weinberger's Department of Health, Education, and Welfare.

The SPLC's complaint shows that the O.E.O. recently began providing funding for such sterilization procedures, while top OEO personnel intentionally did not distribute a medical memo containing guidelines on obtaining patient consent for such operations. Dr. Warren M. Hern authored the memo, and ultimately resigned in outrage that the guidelines were not distributed. Copies of the memo, which included age of consent laws whose criteria the Relf girls did not meet, sat undistributed in a DC warehouse. At the time of the suit, Weinberger's most recent approved Health, Education, and Welfare budget included specific funding allotments for sterilization procedures, and thus he was named a defendant in the case. A district court involved in Relf V. Weinberger hearings found that anywhere from 100,000 to 150,000 poor people were sterilized annually using federal dollars, and some among those sterilized were coerced into the procedures by doctors who threatened to cut off welfare benefits.^[12] The case shone fresh light on numerous^{[example needed]} state sterilization and eugenics programs nationwide and led to compensation funds and settlements for some victims.^{[citation needed]}

Weinberger was vying for Reagan to appoint him as Secretary of State but was given the position of Secretary of Defense instead.^[13]

Weinberger took the lead in implementing a rollback strategy against Soviet communism. In 1984, journalist Nicholas Lemann interviewed Weinberger and summarized the strategy of the Reagan administration to roll back the Soviet Union:

Weinberger had been Secretary of Defense for six years and ten months, longer than anyone except for Robert McNamara and more recently Donald Rumsfeld. After Weinberger left the Pentagon, he joined Forbes, Inc., in 1989 as publisher of Forbes magazine. He was named chairman in 1993. Over the next decade, he wrote frequently on defense and national security issues. In 1990, he wrote Fighting for Peace, an account of his Pentagon years. In 1996, Weinberger co-authored a book entitled The Next War, which raised questions about the adequacy of US military capabilities following the end of the Cold War.

He was a member of the Founding Council of the Rothermere American Institute at the University of Oxford and a fellow at the Institute for Advanced Studies in the Humanities at the University of Edinburgh.^[34]

In 1996, Weinberger was the first host of World Business Review, a television infomercial series.^[35]

While residing on Mount Desert Island, Weinberger was treated for and died from complications of pneumonia at Eastern Maine Medical Center in Bangor, Maine, aged 88. He was survived by his wife, their two children, and several grandchildren.

He is buried in Section 30, Grave 835–1 at Arlington National Cemetery on April 4, 2006.

Shortly after his death George W. Bush in a public statement said:

Caspar Weinberger was an American statesman and a dedicated public servant. He wore the uniform in World War II, held elected office, and served in the cabinets of three Presidents. As Secretary of Defense for President Reagan, he worked to strengthen our military and win the Cold War. In all his years, this good man made many contributions to our Nation. America is grateful for Caspar Weinberger's lifetime of service. Laura and I send our condolences and prayers to the entire Weinberger family.^[44]

Then Secretary of Defense Donald Rumsfeld stated:

Cap Weinberger was a friend. His extensive career in public service, his support for the men and women in uniform and his central role in helping to win the Cold War leave a lasting legacy ... He left the United States armed forces stronger, our country safer and the world more free.^[45]

• Weinberger, Caspar W. Fighting for peace: Seven critical years in the Pentagon (Warner Books, 1990)
• Weinberger, Caspar W. and Peter Schweizer. The next war (Regnery, 1998).
• Weinberger, Caspar W., and Gretchen Roberts. In the Arena: A Memoir of the 20th Century (Regnery Publishing, 2001).
• Weinberger, Caspar W. "Report on Allied Contributions to the Common Defense" (Department of Defense: April 1987) Online Archived October 29, 2019, at the Wayback Machine