MIAMI — Ketanji Brown Jackson does not much talk about it, but when she was a college freshman, an uncle was sentenced to life in prison — a Black man, like so many others, handed a severe punishment during the war-on-drugs era.

The story of Thomas Brown’s cocaine conviction in the rough-and-tumble Miami of the 1980s formed only part of her early understanding of the criminal justice system’s complexities. Another uncle was Miami’s police chief. A third, a sex crimes detective. Her younger brother worked for the Baltimore police in undercover drug stings.

And then there is Judge Jackson, 51, whose peripatetic legal career, guided by the needs of marriage and motherhood, led her to big law firms, a federal public defender’s office, the United States Sentencing Commission and the federal bench, where she is widely seen as a contender to fulfill President Biden’s pledge to nominate the first Black woman to the Supreme Court.

The man she would succeed, Justice Stephen G. Breyer, who announced his retirement last week, once hired her as a clerk and alluded during her 2013 swearing-in ceremony to how her background strengthened her legal foundation.

“She sees things from different points of view, and she sees somebody else’s point of view and understands it,” he said.

Judge Jackson has not yet written a body of appeals court opinions expressing a legal philosophy, having joined the U.S. Court of Appeals for the District of Columbia only last summer.

Her earlier rulings as a district judge in Washington, however, comported with those of a liberal-leaning judge, blocking the Trump administration’s attempts to fast-track deportations, cut short grants for teen pregnancy prevention and shield a former White House counsel from testifying before Congress about President Donald J. Trump’s efforts to obstruct the Russia investigation.

As a judge, she is known more for being detailed and thorough, sometimes to a fault, than for crisp and succinct rulings. Her high-profile opinion in 2018 invalidating Mr. Trump’s executive orders that sought to undermine labor protections for public employees sprawled over 119 pages and peaked with an 84-word sentence.

She tends to assert lively command during arguments and hearings, displaying the skills of a national oratory champion in high school. And on a bench that would have more women than ever, Judge Jackson would bring particular knowledge of criminal law and sentencing legal policy.

That she just underwent a Senate confirmation is seen as another mark in her favor. The Senate confirmed her to the appeals court in June by a 53-to-44 vote. All 50 Democratic caucus members voted for her, as did three Republicans: Susan Collins of Maine, Lindsey Graham of South Carolina and Lisa Murkowski of Alaska.

But it is her rulings over more than eight years on the Federal District Court that have attracted the most attention.

In 2017, she sentenced a man to four years in prison after he had fired a military-style rifle inside a Washington pizzeria. He had been deluded by a false internet conspiracy theory, known as Pizzagate, that Hillary Clinton was operating a pedophile ring there.

After she invalidated Mr. Trump’s executive orders that undercut public labor union protections, an appeals court unanimously reversed her ruling on the grounds that the courts lacked jurisdiction to consider whether the orders were lawful.

In perhaps her most famous decision, Judge Jackson ruled in 2019 that Donald F. McGahn II, the former White House counsel, had to obey a congressional subpoena seeking his testimony about Mr. Trump’s actions during the Russia investigation.

“Presidents are not kings,” she wrote, adding that current and former White House officials owe their allegiance to the Constitution. “They do not have subjects, bound by loyalty or blood, whose destiny they are entitled to control.”

But Judge Jackson also took nearly four months to resolve the case, including writing a 120-page opinion, even though it would inevitably be appealed. That slow pace contributed to helping Mr. Trump run out the clock on the congressional oversight effort before the 2020 election, though the Biden administration later struck a deal for Mr. McGahn to testify behind closed doors.

(By contrast, after Mr. Biden elevated Judge Jackson to the appeals court, she was part of a three-judge panel whose handling of another closely watched case — Mr. Trump’s challenge to a congressional subpoena for White House records related to the Capitol riot — was notably faster. The panel ruled, in an opinion written by a colleague, that Congress could see the documents less than a month after the case appeared on its docket.)

Being a judge was a dream from a young age.

Ketanji Brown was born in Washington and grew up in South Florida, where her parents began as teachers and rose as administrators in the Miami-Dade County Public Schools. Her mother was a principal and her father was the school board’s top lawyer.

One of young Ketanji’s earliest memories was sitting side by side in the evenings with her father when he was in law school — him with law books, her with coloring books.

“There really is no question that my interest in the law began that early on,” she said at a 2017 lecture at the University of Georgia, where she spoke candidly about the obstacles facing female lawyers, especially those of color.

She excelled at Miami Palmetto Senior High — alumni include the Amazon founder Jeff Bezos — and was a star on its powerhouse speech and debate team. In her senior yearbook, she said she hoped to someday get a judicial appointment.

“She had extraordinary talent,” said Amy Berger Chafetz, who was on the debate team with her.

A debate competition took her to Harvard, where she fell in love with the university, despite winters she called “unbearable.” As an undergraduate, she joined an improv group and took a drama class where one scene partner was Matt Damon.

Her “serious boyfriend,” a pre-med student named Patrick G. Jackson, was a “Boston Brahmin,” she said in the lecture — a sixth-generation Harvard graduate with roots dating back to England before the Mayflower. By contrast, her family descended from slaves, and she was only the second generation to graduate from college.

After her 1992 graduation, she spent a year as a reporter at Time magazine before returning to Cambridge for Harvard Law School. In 1996, she earned her law degree and married Dr. Jackson. They now have two daughters, Talia, 21, and Leila, 17. Dr. Jackson is a general surgeon at MedStar Georgetown University Hospital.

Ms. Jackson clerked for several judges, including Justice Breyer. She worked in his chambers during the 1999-2000 term, when the court considered issues like gay rights, public prayers at high school football games and so-called partial-birth abortion.

A brief Boston law firm stint proved untenable. “You start to feel as though the demands of the billable hour are constantly in conflict with the needs of your children,” she said in the lecture.

The family returned to Washington, where Ms. Jackson joined the federal public defender’s office. She worked largely on appeals but also on behalf of several men indefinitely detained without charges at Guantánamo Bay, Cuba.

During her 2017 appeals court confirmation hearing, Senator Tom Cotton, Republican of Arkansas, challenged her about that work.

Judge Jackson told him she had been assigned those cases and noted that her brother was deployed to Iraq with the military. In a written follow-up response, she portrayed herself as one of “many lawyers who were keenly aware of the threat that the 9/11 attacks had posed to foundational constitutional principles, in addition to the clear danger to the people of the United States.”

Later, as an associate at a corporate law firm, Ms. Jackson also filed friend-of-the-court briefs on behalf of two groups supporting challenges to Bush-era detention policies, including a claim that the government could detain a lawful permanent resident arrested on American soil without charges and as an enemy combatant.

In 2009, President Barack Obama nominated her to the Sentencing Commission. It made a major decision during her tenure to back retroactively lightening some sentences for past crack cocaine convictions.

Ms. Jackson was adept at finding common ground among the seven commission members — four appointed by Democrats, three by Republicans — said Rachel E. Barkow, a law professor at New York University who served with her.

“I think that would be very helpful to her moving to a nine-member body and trying to get consensus across the aisle,” she said, referring to the Supreme Court.

In September 2012, Mr. Obama nominated Ms. Jackson to serve as a district court judge. But the timing was fraught: The Senate did not have time to confirm her before his term ended, meaning her fate would hinge on whether he was re-elected.

At the same time, however, she was related by marriage to the Republican vice-presidential nominee: Paul D. Ryan, the House speaker. The twin brother of her husband, Dr. Jackson, is married to the sister of Mr. Ryan’s wife.

She knitted to relieve the stress, she said in the Georgia lecture: “I was unusually jumpy and started so many scarves that I could have outfitted a small army.”

Mr. Obama was re-elected, and at her December 2012 confirmation hearing, Mr. Ryan testified in her support, calling her “an amazing person” and “clearly qualified.”

“Our politics may differ, but my praise for Ketanji’s intellect, for her character, for her integrity, it is unequivocal,” Mr. Ryan told the Senate Judiciary Committee.

During her confirmation process, Ms. Jackson lauded her police officer relatives. But she does not appear to have spoken openly then or in other public events about Mr. Brown, her uncle who went to prison.

He was sentenced to life in October 1989 for possessing a large amount of cocaine with intent to distribute it. He was released in November 2017, after Mr. Obama commuted most of his remaining sentence, along with those of many others sentenced when so-called three-strikes laws sent many nonviolent drug offenders to prison for decades. Public records suggest Mr. Brown died about four months later.

Two pieces pegged to Judge Jackson’s appellate nomination last year, in The Washington Post and on NPR, mentioned her relationship to Mr. Brown in passing, without citing the source of the information. Last week, Judge Jackson declined through a representative to comment about Mr. Brown. But two people familiar with the matter, speaking on the condition of anonymity because of its sensitivity, confirmed the relationship.

Still, Judge Jackson has hinted at her deep engagement with the criminal justice system in other ways. During her confirmation process last year, a Republican senator asked whether she had been concerned that her work as a public defender could put violent criminals back on the streets. (No sitting Supreme Court justice has a background in public defense.)

In a written response, Judge Jackson said that competent defense lawyers were important to making the system function.

“Having lawyers who can set aside their own personal beliefs about their client’s alleged behavior or their client’s propensity to commit crimes benefits all persons in the United States,” she wrote, “because it incentivizes the government to investigate accusations thoroughly and to protect the rights of the accused during the criminal justice process.”

In the aggregate, she added, that “reduces the threat of arbitrary or unfounded deprivations of individual liberty.”

Patricia Mazzei reported from Miami, and Charlie Savage from Washington. Elizabeth Williamson contributed reporting from Washington. Kitty Bennett and Jack Begg contributed research.