Weinberger was born in San Francisco, California, the younger of two sons of Herman Weinberger, a Colorado born lawyer. His mother, the former Cerise Carpenter Hampson, was an accomplished violinist whose parents were immigrants from England. Weinberger was named "Caspar" for a friend of his mother's; his father began calling him "Cap", a nickname that stuck into adulthood.

Weinberger was a first cousin of the nationally broadcast radio personality Don McNeill of Don McNeill's Breakfast Club; their mothers were sisters - in - law. Caspar Weinberger's father Herman was the younger brother of Luella Weinberger McNeill, mother of Don McNeill. The 1910 Census shows Herman and Luella living in the household of Nathan Weinberger, the grandfather of Caspar Weinberger. Weinberger was a sickly child and required close nurturing from his mother; in time, he overcame his poor health and shyness.

Weinberger's paternal grandparents had left Judaism because of a dispute at a Bohemian synagogue. He was reared in a home with no denominational ties, though with a general Christian orientation. In time he became an active Episcopalian and often expressed his faith in God. When he enrolled at Harvard University in Cambridge, Massachusetts, Mrs. Weinberger rented an apartment nearby for the first semester that Weinberger and his older brother, Peter, attended Harvard. She then returned to her husband in San Francisco. Weinberger received his Bachelor of Arts, magna cum laude, in 1938 and a Juris Doctor degree in 1941, both from Harvard. He edited the Harvard student newspaper, The Harvard Crimson, and recalls in his memoirs entitled In the Arena: A Memoir of the 20th Century two specific interviews of which he was most pleased: one with the highly decorated soldier Theodore Roosevelt, Jr., and another with Alabama born actress Tallulah Bankhead.

He entered the United States Army as a private in 1941, was commissioned as a second lieutenant at the United States Army Officer Candidate School in Fort Benning, Georgia, and served with the 41st Infantry Division in the Pacific. At the end of the war he was a captain on General Douglas MacArthur's intelligence staff. Early in life, he 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 - 1947, Weinberger worked as a law clerk for a federal judge before joining a San Francisco law firm.

Weinberger entered the race for California assemblyman in the San Francisco Bay area in 1952 at the persuasion of his wife, Jane Weinberger, who also served as his campaign manager. He won election to the California State Assembly in 1952 and reelection 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. 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 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 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.

Although not widely experienced in defense matters, Weinberger had a reputation in Washington as an able administrator; his powers as a cost cutter earned him the sobriquet "Cap the Knife." He shared President Reagan's conviction that the Soviet Union posed a serious threat to the United States, and that the defense establishment needed to be modernized and strengthened. Belying his nickname, at the Pentagon Weinberger became a vigorous advocate of Reagan's plan to increase the Department of Defense budget. Readiness, sustainability and modernization became the watchwords of the defense program. In his early years at the Pentagon, Cap Weinberger was known as "Cap the Ladle" for advocating large increases in defense spending.

As Secretary of Defense, Weinberger oversaw a massive rebuilding of US military strength. Major defense programs he championed included the B-1B bomber and the "600 ship Navy". His efforts created economic and military - industrial pressures that were associated with the beginning of Perestroika and the beginning of the end of both the Cold War and the Soviet Union.However this thesis was not confirmed by the extensive study on the causes of the collapse of the Soviet Union by two prominent economists from the World Bank - William Easterly and Stanley Fischer from the Massachusetts Institute of Technology. “… the study concludes that the increased Soviet defense spending provoked by Mr. Reagan's policies was not the straw that broke the back of the Evil Empire. The Afghan war and the Soviet response to Mr. Reagan's Star Wars program caused only a relatively small rise in defense costs. And the defense effort throughout the period from 1960 to 1987 contributed only marginally to economic decline."

While these events were clearly substantial and world changing, they came at the cost of helping to triple the national debt. Weinberger pushed for dramatic increases in the United States' nuclear weapons arsenal and was a strong advocate of the controversial Strategic Defense Initiative, an initiative which proposed a space and ground based missile defense shield.

In response to the 1983 Beirut barracks bombing, President Reagan assembled his national security team and planned to target the Sheik Abdullah barracks in Baalbek, Lebanon, which housed Iranian Revolutionary Guards believed to be training Hezbollah fighters. However Weinberger said there would be no change in the US's Lebanon policy and aborted the mission, reportedly because of his concerns that it would harm US relations with other Arab nations.

Though he claims to have been opposed to the sale on principle, Weinberger participated in the transfer of United States TOW anti - tank missiles to Iran during the Iran – Contra affair. By 1987, the disclosure of the Iran - Contra Affair and increasing difficulties with Defense budgets weighed on Weinberger. Weinberger resigned on November 23, 1987, citing his wife's declining health. He specifically denied that he was opposed to the INF Treaty, scheduled to be signed in Washington in December 1987. In fact, he took credit for proposing the substance of the treaty early in his term at the Pentagon.

Following his resignation as Secretary of Defense, legal proceedings against Weinberger were brought by Independent Counsel Lawrence E. Walsh. A federal grand jury indicted Weinberger on two counts of perjury and one count of obstruction of justice on June 16, 1992. He was defended by defense attorney Carl Rauh. Weinberger received a Presidential pardon from George H. W. Bush on December 24, 1992.

Weinberger had been Secretary of Defense for six years and ten months, longer than any man except for Robert McNamara and 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.

In 1942 Weinberger married Rebecca Jane Dalton, who was born on March 29, 1918 in Milford, Maine. A World War II Army nurse, and later author and publisher, she "[c]oaxed her husband ... into politics and was a loyal Washington wife during three Republican administrations before she began to write and publish children's books". Jane Weinberger, a uterine cancer survivor, died on July 12, 2009, aged 91, at Bar Harbor, Maine, following a stroke. The couple had a daughter, Arlin, and a son, Caspar.

While residing on Mount Desert Island, Maine, 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, the former Rebecca Jane Dalton, their two children and several grandchildren.

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

Shortly after his death President 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.

Then United States 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.

• Presidential Medal of Freedom in 1987.
• Honorary Knight Grand Cross of the Order of the British Empire in 1988, awarded in recognition for an "outstanding and invaluable" contribution to military cooperation between the UK and the US, particularly during the Falklands War of 1982.
• Weinberger was awarded the Gold Star Award by the International Strategic Studies Association for Outstanding Contributions to Strategic Progress Through Humanitarian Achievement.
• Caspar Weinberger was inducted into the United States Army Officer Candidate School Hall of Fame in 1981, the highest honor that can be bestowed on any graduate from the school.