As a young prosecutor in the late 1990s, Sally Q. Yates sat at a conference table with a former sheriff and began picking away at his story. With an F.B.I. agent watching, Ms. Yates soon had the lawman in knots about a deposition.

“I watched him as she broke him down, and he confessed that he had lied under oath,” the agent, Oliver G. Halle, now retired, recalled on Tuesday. “She can be very disarming, but underneath that disarming appearance is a woman who knows how to fight.”

As acting attorney general, Ms. Yates picked the fight of her life on Monday when she ordered the Justice Department not to defend President Trump’s executive order blocking refugees and restricting immigration to the United States. Ms. Yates became convinced, based on the president’s own statements, that he had intended to unlawfully single out Muslims, senior officials said.

“We have comments from the president about what this is supposed to do,” Ms. Yates said in one meeting on Monday, according to two people involved in the discussions. She later added, “The intent was clear from the face of it.”

Her firing was a politically divisive turn in a career that had, until now, earned her bipartisan praise.

“She will be a hero of the American people, a hero of what’s right,” Senator Johnny Isakson, Republican of Georgia, said in 2015 at Ms. Yates’s confirmation hearing. “She’ll call them like she sees them, and she will be fair, and she will be just.”

While Ms. Yates was a reliably liberal voice in the Justice Department on issues of civil rights, criminal justice and sentencing, she worked her way up as a career prosecutor in Atlanta under political appointees from both parties. A native Georgian, she led prosecutions against some of the highest profile defendants in Atlanta, including former Mayor Bill Campbell, a Democrat who was accused of racketeering and tax fraud, and Eric Robert Rudolph, who set bombs at a park during the 1996 Olympic Games, a gay nightclub and two Southern abortion clinics.

She also took on the leaders of an Atlanta suburb who refused to allow construction of a mosque. The Justice Department sued, and the city reversed itself. “Religious freedom requires that local government decisions impacting the exercise of that freedom be free of discrimination,” Ms. Yates said at the time.

When Ms. Yates, who declined to comment on Tuesday, became deputy attorney general in 2015, she told colleagues that she had no intention of merely being a caretaker. “We’re going to run through the tape,” she often said.

In Washington, her outgoing personality made her a counterpoint to her more reserved boss, Attorney General Loretta E. Lynch. At times that made Ms. Yates the face of the Justice Department in ways that caused tension with Ms. Lynch’s staff. Ms. Yates was regarded as professionally ambitious, though she has told friends that she has no interest in running for political office.

Last year, Ms. Yates and Ms. Lynch earned the ire of Democrats — including many in the department — for not intervening to prohibit the F.B.I. director, James B. Comey, from sending a letter to Congress in the final days of the presidential campaign. The letter raised the prospect of new and potentially damaging evidence against Hillary Clinton related to an investigation that had been closed. Nothing came of the new evidence, and Mrs. Clinton’s team says the letter cost her the presidency. Her supporters argued that Justice Department leaders were too timid to stand up to Mr. Comey.

Mr. Trump’s executive order prompted a new challenge for Ms. Yates, who was serving until the Senate confirmed a new attorney general. The Justice Department’s Office of Legal Counsel had reviewed and signed off on the order, but Ms. Yates believed that the department had to also consider the president’s intent, which she said appeared aimed at singling out people based on religion.