MIAMI — In her just over 20 months as a federal judge, Aileen M. Cannon worked mostly in obscurity, becoming nominated and appointed to her position during the height of the coronavirus pandemic and at the end of a turbulent presidency.

Then, last month, she was assigned the most prominent case of her short judicial career, involving the very person who put her on the bench: former President Donald J. Trump.

On Monday, Judge Cannon granted Mr. Trump’s request to appoint an independent arbiter known as a special master to review materials seized last month from his private Florida club. The extraordinary and unusually broad decision, which could delay the criminal investigation into Mr. Trump, drew scrutiny from experts who questioned her legal reasoning and criticized some of the language in her opinion about what rights a former president is entitled to.

William P. Barr, who was attorney general under Mr. Trump, took exception to her ruling, saying that the judge did not adequately address a key issue in dispute: whether a former president may invoke executive privilege to keep the executive branch itself from reviewing documents while investigating a potential crime. He said the answer is no.

“The opinion, I think, was wrong,” Mr. Barr said on Fox News on Tuesday. “And I think the government should appeal it. It’s deeply flawed in a number of ways.”

Little is publicly known about Judge Cannon, 41, whose name quickly became familiar after her ruling during the holiday weekend. She joined the conservative Federalist Society as a law student in 2005 and maintained her ties to the group as her career unfolded, a fact that she made public during her Senate confirmation hearings in 2020. But according to people involved in the group’s activities, she was not an especially visible presence.

At the time of her nomination, Ms. Cannon had been a lawyer for 12 years, the minimum threshold to meet the American Bar Association’s qualification standard. Most of her career was spent as a federal prosecutor, though she had limited trial experience because she focused on appellate work.

As a judge, she had not overseen cases that attracted much attention before she was assigned Mr. Trump’s high-profile lawsuit. She got the case after Mr. Trump avoided visiting the issue with the magistrate who approved the search of his Mar-a-Lago estate.

Many of her hearings in the Southern District of Florida were at first handled via Zoom. And she works out of a courthouse in Fort Pierce, which has its share of routine drug and immigration cases but is generally a far quieter part of the region than bustling Miami.

“It’s usually like walking into a mausoleum up there,” Donnie Murrell, a criminal defense lawyer in West Palm Beach, said of the federal courthouse in Fort Pierce, an imposing structure that opened in 2011. “You hear footsteps echoing when you walk.”

Valentin Rodriguez Jr., a defense lawyer based in West Palm Beach who worked opposite Ms. Cannon when she was a prosecutor and has appeared before her as a judge, said she was thorough, meticulous and often willing to rule against the government, as she did in Mr. Trump’s case.
“The general feeling that I’ve gotten from her is, ‘I don’t buy everything the government has to tell me,’” Mr. Rodriguez said. “You can’t expect that if you and the government have some sort of agreement, over sentencing or a plea, that that’s necessarily going to convince. In that sense, you could call her something of a freethinker.”

Judge Cannon went to lengths to allow Mr. Trump’s legal team to clarify its argument after an initial filing that was too vague. During a hearing in the Trump case last week, she also seemed to help one of Mr. Trump’s lawyers remember that his client’s request for a special master included not only to review documents under attorney-client privilege but also to assess any that could be covered under executive privilege.

Aileen Mercedes Cannon was born in Cali, Colombia, but grew up in Miami along with an older sister. Her mother, Mercedes Cubas, fled Cuba as a young girl after the 1959 Communist revolution. The family of her father, Michael Cannon, hailed from Indiana.

In cliquish Miami, where high school connections can run deep, she graduated from Ransom Everglades, a private school on the shores of Biscayne Bay in Coconut Grove. She swam, played water polo and was known as popular and studious.

“Aileen was always an incredibly dedicated and diligent student,” said Alejandro Miyar, a lawyer who worked for the Obama administration. He was one of 17 Ransom graduates who signed a letter in 2020 supporting Ms. Cannon’s nomination.

The letter described her as “personable and trustworthy, a genuinely caring person who treats others as she would want to be treated herself.”

“What more can we ask of another human being?” it read.

Ms. Cannon graduated from Duke University, spending a semester in Spain and a summer writing short feature articles for El Nuevo Herald, a daily Spanish-language newspaper, then graduated from the University of Michigan Law School.

In 2008, she married Josh Lorence, who is an executive for Bobby’s Burgers, the celebrity chef Bobby Flay’s fast-casual restaurant chain, according to his LinkedIn profile, which was no longer publicly viewable on Monday. He proposed while they were on vacation in Greece. They have two children and live in Vero Beach, along Florida’s Treasure Coast. Public records show that Ms. Cannon has registered as a Republican. In 2018, she and her husband each contributed $100 to Ron DeSantis’s campaign for governor.

Through his office, Senator Marco Rubio, Republican of Florida, reached out to Ms. Cannon in 2019 about filling a judicial vacancy, she said in her questionnaire to the Senate Judiciary Committee.

Howard Srebnick, a Miami lawyer who went to the same high school as Judge Cannon, said she had all the necessary credentials to be a federal judge. She worked as a federal prosecutor, clerked for a conservative federal judge and spent time in a large law firm, Gibson, Dunn & Crutcher, in Washington, where she was known as a quiet presence who disliked attention.

“I don’t think anyone could say she’s professionally or intellectually unqualified,” Mr. Srebnick said.

He added that as a prosecutor he found Ms. Cannon to be polite and respectful of defense lawyers — a trait that not all prosecutors share.

“As a judge,” he said, “you may not agree with her decisions, but she is always respectful of the process.”

As a clerk, she worked for Judge Steven M. Colloton, who sits on the U.S. Court of Appeals for the Eighth Circuit in Iowa and was at one point on Mr. Trump’s list of possible Supreme Court nominees.

At her Senate Judiciary Committee nomination hearing in July 2020, Ms. Cannon appeared via Zoom, framed by an American flag and her academic degrees. She recounted how her mother fled Cuba at the age of 7 and said her maternal grandparents “were forced to leave everything they had.”

“They taught me always to be thankful for this country and to cherish our constitutional democracy,” she said.

Charlie Savage contributed reporting. Kitty Bennett contributed research.