The senator was hailed as a pioneer in politics. Here’s what to know.

“She has a level of integrity that’s unusual — she really thinks about the merits, more than most,” said Jerry Brown, 84, a fellow Democrat who was California’s longest-serving governor when he left public office in 2018. “She gives her all to the job. And I think it has been difficult for her to come to the end.”

Jan. 15, 1962

A Political Career Begins

In 1955, Dianne Goldman, the oldest child of a well-known San Francisco physician and his wife, a former model, graduated from Stanford University. She did not have the grades to follow her father into medicine, she would write later, but she had excelled in student government.

She won a Coro Foundation internship in the San Francisco district attorney’s office, telling interviewers that she planned to run soon for elective office. Instead, she married a prosecutor and had a daughter. By 1960, she was unemployed and divorced.

Edmund G. “Pat” Brown, the governor of California and a patient of her father’s, appointed her to a $600-a-month post on the state parole board for women, citing, among other qualifications, her Coro report on criminal justice. In a 1994 account of her early years, “Dianne Feinstein: Never Let Them See You Cry,” Jerry Roberts wrote that the post forced her to immerse herself in the real lives of incarcerated women and shaped her views on abortion, capital punishment and law and order.

It also gave her a platform: When she published a series of articles on criminal justice reform in one of the San Francisco newspapers, a civil rights lawyer on the board of supervisors named her to a watchdog panel on jail conditions. When she made headlines there with a report on vermin and “glutinous” jail meals, Joseph Alioto, the mayor of San Francisco, appointed her to a blue-ribbon crime committee. And by 1969, San Franciscans knew the young matron with the good government credentials and the Jacqueline Kennedy bob.

Nov. 4, 1969

President of the San Francisco Board of Supervisors

Remarried by now to Dr. Bert Feinstein, a neurosurgeon, Dianne Feinstein launched her campaign for the 11-member board of supervisors that governs the city. Her moderate agenda would become a hallmark: lower crime, “responsible” taxation and spending, environmental conservation, and a more efficient government.

The city was in flux, disrupted by the Summer of Love and torn by societal changes. The local press focused on better-known male incumbents. Fellow Democrats viewed her as an outsider among the party’s vying factions. The only elected official to endorse her was a State Assembly member she knew from local civil rights marches, Willie Brown Jr., who would eventually become one of the most powerful politicians in the state.

To compensate, she courted outsiders — the politically nascent gay community and opponents of the high rises that were transforming the city. And, in another future hallmark, she raised prodigious sums from wealthy friends and family and advertised heavily on TV. Young, telegenic and new, she not only upset the field in a landslide, but as the highest-vote getter, she automatically became board president, the first woman to do so in city history.

Nov. 27, 1978

The Assassinations of George Moscone and Harvey Milk

By the 1970s, violence and political tumult had polarized the city. The racially motivated Zebra killings left 15 people dead. The Symbionese Liberation Army kidnapped Patty Hearst, the heiress of the city’s newspaper publisher, from her apartment in nearby Berkeley. Leftist terrorists bombed scores of targets, including one attempt at Supervisor Feinstein’s home.

Calls for gay rights on one side of town had engendered conservative backlash on the other. Caught in the middle, Ms. Feinstein — derided by her male political rivals as “Goody Two Shoes” — ran twice for mayor and lost.

By late 1978, cancer had claimed the lives of both her father and husband. Her daughter by now was 21. Back to work after a trip to Nepal with Richard Blum, the wealthy investment banker who would become her third husband, Ms. Feinstein casually told City Hall reporters that she was thinking of leaving public service. She was 45.

Hours later, shots exploded down the hall from her office. Dan White, a conservative former supervisor who had resigned from the board and had been unable to get his job back, had killed Mayor George Moscone and Harvey Milk, the openly gay supervisor who had advised the progressive mayor against reinstating him.

Ms. Feinstein rushed to help Mr. Milk; when she reached for a pulse, her fingers slipped into a bullet hole, she would later remember. Mr. Moscone’s death meant that Ms. Feinstein, as board president, was now automatically acting mayor. Ashen and bloodstained, she faced the cameras, reporting the assassinations and declaring a period of mourning. In an editorial, The San Francisco Chronicle praised her reassuring poise under pressure. A week later, the surviving liberals on the board — who previously had spurned her centrism — rallied behind her and elected her to finish Mr. Moscone’s mayoral term.

November 1978 — January 1988

Serving as Mayor of San Francisco

Ms. Feinstein later wrote that she learned her “greatest political lesson” during her nine years as mayor. “It’s not about being ‘politically correct,’” she wrote in “Nine and Counting: The Women of the Senate,” a collection of observations on leadership by female politicians, “but about what works and solves the problem at hand.”

She professionalized staff meetings, enlarged the police force and was pointedly visible in performing nuts-and-bolts mayoral duties. She would show up at fires and crime scenes, sometimes personally evacuating people or administering mouth-to-mouth resuscitation. She donned coveralls and painted over graffiti on weekend cleanup patrols.

She was re-elected twice, but leading from the center cost her. Gay rights advocates praised her support of programs to fight AIDS and protect employment and housing, but were infuriated when she vetoed early domestic partner legislation that conservatives opposed.

Women’s groups touted the city’s first female mayor, but complained that she refrained from identifying herself closely with the movement. A refurbishment of the cable car system won applause, but a downtown construction boom led critics to accuse her of being too pro-business. In 1983, when pro-gun communists sought to recall her over the city’s short-lived but historic ban on handguns, there was enough disenchantment to bring their petition to a vote.

Defeating the recall left her stronger, however. In 1984, when San Francisco hosted the Democratic National Convention, Walter Mondale, the Democratic presidential nominee, considered her as his running mate. He ultimately chose Geraldine Ferraro, and President Ronald Reagan won the general election, but the process set her up to run in 1990 for California governor.

Nov. 3, 1992

The First Female Senator From California

Ms. Feinstein won the Democratic primary in the race to succeed Gov. George Deukmejian in a come-from-behind campaign that was financed significantly by her husband. If Californians did not know her as a moderate before, that race cemented her reputation; statewide ads featured her taking charge after the Moscone and Milk assassinations and being booed at the state Democratic Party convention while declaring her support for capital punishment.

She narrowly lost the election to Senator Pete Wilson, a Republican who shared her support for abortion rights, environmental protections and the death penalty. But two years later, she won a special election to fill the Senate vacancy left by Mr. Wilson’s departure from Congress, defeating John Seymour, a legislator who had been appointed in the interim to replace him.

The race took place in an election cycle that was fueled by outrage over the Supreme Court confirmation hearings of Justice Clarence Thomas by the male-dominated Senate, during which Anita Hill testified that Mr. Thomas had sexually harassed her when he was her supervisor. In what would come to be called “The Year of the Woman,” Senator Feinstein was sworn in shortly after the election, two months ahead of Senator Barbara Boxer, giving California two female senators.

After the tumultuous Thomas confirmation hearings, the chairman of the Judiciary Committee, Senator Joseph R. Biden Jr., said he wanted a woman on the panel to put to rest any accusations of insensitivity among its membership of 14 white men. And in 1993, Ms. Feinstein, along with Senator Carol Moseley Braun of Illinois, was appointed to the committee, giving her significant sway over future appointees to the Supreme Court.

Sept. 13, 1994

Shaping the Federal Assault Weapons Ban

In the early 1990s, after three high-profile shootings over the course of five years — two of them in California — federal lawmakers began pushing for a ban on assault-style weapons. Ms. Feinstein took a lead role in the effort, eventually writing a bill, often called the Federal Assault Weapons Ban, that passed the Senate by 91 votes.

The ban was supported by three former presidents, including Ronald Reagan, and eventually passed in the House by 40 votes. The final bill, enacted in 1994, outlawed 14 semiautomatic assault weapons, including Kalashnikov- and AR-15-style rifles, as well as those with functions, such as high-capacity magazines, that allowed them to be converted into military-style weapons. Ms. Feinstein considered it a watered-down version of the bill she had intended, but it was seen by many as a major legislative achievement.

One political concession was that the bill would expire in 2004 if not renewed, which it was not. Experts have since argued about the effectiveness of the ban, with some saying it decreased gun violence, others saying it had no impact and still others saying it was hampered by loopholes. Ms. Feinstein, and other Democrats, continued the attempt to renew the ban — most recently in 2022 — but failed to do so.

Dec. 9, 2014

The Senate Intelligence Committee Report on C.I.A. Torture

In late 2007, The New York Times reported that the Central Intelligence Agency had secretly destroyed tapes of interrogations conducted in 2002 of two Al Qaeda operatives that the agency had in its custody.

The tapes reportedly showed agency operatives subjecting the terror suspects to severe interrogation techniques, such as waterboarding. Agency officials said the tapes had been destroyed because they might endanger undercover officers, their intelligence value had ended and they might expose the agency to legal jeopardy.

The Senate Intelligence Committee, of which Ms. Feinstein was a member, began an investigation. The report provided to the committee a little more than a year later described the extensive torture of Abu Zubaydah, whom the C.I.A. suspected was a high-ranking Al Qaeda member, and of Abd al-Rahim al-Nashiri, another detainee.

By the time committee members received the report in early 2009, Ms. Feinstein, then 75, was the committee chairwoman, and she called for a full investigation of the C.I.A.’s detention and interrogation program. The project — and the senator’s fight with the Obama administration to release what the committee’s staff had uncovered about the program’s brutality, inefficiency and needless secrecy to the public — consumed more than five years.

Drawing on millions of C.I.A. documents, the full, 6,700-page “torture report,” as it came to be known, remains classified. But the 500-page executive summary, issued in 2014 despite a relentless campaign by the C.I.A. to undermine it, delivered a sweeping indictment of the agency and its treatment of terrorism suspects in the years after the Sept. 11 attacks. So epic was the fight to inform Americans that a movie was made about it in 2019, with Annette Bening playing Ms. Feinstein.

Over the decades, Ms. Feinstein’s accomplishments ranged from the federal coordination of Amber Alerts, the national child abduction warning system, to the California Desert Protection Act, which preserved more than 7 million acres of desert. But she called the torture report the most important work of her career.

September 2018

Confirmation Hearings of Brett Kavanaugh

In 2018, while facing her first competitive re-election race in decades, Ms. Feinstein weathered some of the worst criticism of her Senate career during the contentious Supreme Court confirmation hearings for Justice Brett Kavanaugh, who was nominated by President Donald J. Trump.

At the time the top Democrat on the Judiciary Committee, Ms. Feinstein had received a letter accusing Justice Kavanaugh of sexual assault but held onto it privately for weeks. Then, a week before the nomination vote, she acknowledged its existence, propelling Christine Blasey Ford to reluctantly come forward with her accusation.

President Trump and Republican lawmakers excoriated Ms. Feinstein for her handling of the matter. They raised doubts about Dr. Blasey’s account and accused the Democratic senator of meddling with the proceedings. Ms. Feinstein said that she had kept the letter confidential at Dr. Blasey’s request.

Amid the proceedings, liberal Democrats also raised concerns that Ms. Feinstein was overly deferential toward Justice Kavanaugh, such as when she apologized to him for protests that erupted in the hearing room.

A month after the Senate confirmed Justice Kavanaugh, Ms. Feinstein won re-election to the Senate by just 8 points — her narrowest margin of victory in years.

October 2020

Confirmation Hearings of Amy Coney Barrett

In 2020, as confirmation hearings closed for Judge Amy Coney Barrett — another Trump nominee to the Supreme Court — Senator Feinstein hugged Senator Lindsey Graham, the Republican from South Carolina who led the proceedings. She thanked him and praised his work.

“This has been one of the best set of hearings that I’ve participated in,” she said at the time.

After the confirmation hearings, many Democrats seethed at Ms. Feinstein’s congenial attitude and what they considered meek questioning of Justice Barrett. They were already upset over the death of Justice Ruth Bader Ginsburg, a liberal icon, just before the November election in 2020 and had insisted that Republicans should not replace her until voters had decided on the next president and the balance of power in Congress.

A little more than a month later, Ms. Feinstein stepped down from her position as the top Democrat on the powerful Senate Judiciary Committee at the urging of liberals.

The move came to symbolize a growing frustration among Democrats in an ever-polarized age that Ms. Feinstein, after decades in power, would not sufficiently fight Republican efforts to roll back abortion access and other rights. By February 2022, she had suffered her worst job performance rating yet among California voters.

Still, her friends said, she will be a hard act to follow.

“There’s always somebody, particularly among the younger crowd, who believes that age should have something to do with your continued participation,” said Willie Brown Jr., 88, who served as mayor of San Francisco after his stint in the Assembly and still remains politically influential. “I can’t imagine life without politics and I can’t imagine it for someone who has been in it as long as Dianne has.”

Eliza Fawcett, Erik Vance and Jill Cowan contributed reporting.