WASHINGTON — When Attorney General Merrick B. Garland had to decide whether the Justice Department should continue its highly unusual defense of former President Donald J. Trump in a defamation lawsuit, he relied on the studious approach that had served him as a federal judge for more than two decades.

The case was politically perilous and discomfiting. Mr. Trump had declared that a woman who accused him of rape, the writer E. Jean Carroll, was lying to generate publicity. Mr. Garland later told reporters that if the Carroll matter were a policy question, it was one he would “abhor.” But as attorney general, he had to set aside his personal views to weigh the law.

Mr. Garland set to work. He reviewed the argument built by the department’s lawyers, even reading the relevant cases himself rather than relying only on aides to brief him, according to multiple people briefed on the case. Mr. Garland ultimately decided last month not to overturn the Trump-era decision — that because Mr. Trump made the comments in the capacity of his office, a Justice Department defense was proper — based largely on multiple precedents.

The backlash was swift. Democrats decried the result, and the White House quickly distanced itself from the decision. But Mr. Garland saw it as the kind of impartial call that ensures that the same rules apply both to Democrats and to Republicans. And like many other former prosecutors, he viewed it as best to let the legal filing speak for itself.

In his first months as attorney general, an early challenge is coming into focus for Mr. Garland: to convince the public that a post-Trump Justice Department is not the same as an anti-Trump one.

Mr. Garland, slight, owlish, soft-spoken and deliberative, has had to quickly learn that the scholarly approach that served him as a judge cannot on its own restore independence and credibility to an agency battered by Mr. Trump’s attempts to improperly wield its power. He has struggled to convey his message, according to former law enforcement officials, watching as his agenda was repeatedly subsumed by headlines about politically fraught matters left over from the Trump administration.

“The question for Garland is whether righting the ship of the department is mutually exclusive from moving on from Trump,” said Barbara Perry, a presidential historian at the University of Virginia’s Miller Center.

The decision in the Carroll case was among a spate of Justice Department moves in Mr. Garland’s first months in office that critics — many on the left — denounced as unnecessarily shielding Mr. Trump and his top officials. The department also moved to keep secret a memo that former Attorney General William P. Barr relied on to clear Mr. Trump in the Russia investigation, and Mr. Garland was reticent when Democrats on the House Intelligence Committee asked for more information about why prosecutors seized their account information from Apple, citing longstanding norms that the department does not discuss investigations.

Mr. Garland has also reckoned with prosecutors’ fight to obtain reporters’ phone and email records, including imposing gag orders on media companies. Begun during the Trump administration, that effort spilled over into Mr. Garland’s tenure, and he took responsibility in recent discussions with news executives, according to two people briefed on the conversations.

Together, the moves ended any honeymoon period for Mr. Garland, whom Mr. Biden had hailed as a nominee who could restore the Justice Department’s integrity and independence after it was called into question under Mr. Trump.

Mr. Garland, who declined through a spokesman to answer questions for this article, has acknowledged the difficulty of applying the law without incurring partisan rancor. “Sometimes it means that we have to make a decision about the law that we would never have made and that we strongly disagree with as a matter of policy,” he said during a Senate hearing last month.

He and his supporters have compared his challenge to that of Edward Levi, the post-Watergate attorney general who restored confidence in the department after Richard M. Nixon abused it for personal gain.

But there are major differences. Mr. Nixon left public life in disgrace, disavowed by his own party. And President Ford’s highly unpopular decision to pardon him saved the Justice Department from the divisive decision of whether to prosecute him.

Republicans today have stood by Mr. Trump. And both his allies and his foes remain fixated on whether attempts to hold him accountable for his norm-busting actions will be successful, making his legal matters a referendum on the justice system at large.

To be sure, most of Mr. Garland’s decisions have aligned with Mr. Biden’s agenda. The department is investigating whether police departments in Minneapolis and in Louisville, Ky., systematically discriminate against Black people. It secured federal indictments against four former Minneapolis police officers, including Derek Chauvin, in the death of George Floyd. And it sued Georgia over a new voting law, alleging that it is intended to keep Black voters from the polls.

It overturned Trump-era decisions that limited asylum claims for victims of domestic abuse and gang violence. And it ended a criminal inquiry and civil suit against Mr. Trump’s onetime national security adviser John R. Bolton that was widely seen as retaliation by Trump officials for Mr. Bolton’s damning memoir about his time in the White House.

In early meetings as attorney general, Mr. Garland asked granular questions about the statutes used to support pending matters and cases that the department had already been pursuing, according to people briefed on the discussions. While he has been less probing as the Senate has confirmed more members of his senior leadership team, his early leadership style would be familiar to his former clerks, who came to expect a long string of precise, detail-oriented questions when working on any brief.

“He always finds places where we glossed over the facts too quickly or summarized an argument in a way that was not quite right,” said Jonathan Kravis, a former federal prosecutor who also clerked for Mr. Garland when he was a judge. “At the time, it drives you nuts, but you learn the lesson of getting the details right.”

Mr. Garland is often described as a centrist, but people who know him well say that he is guided more by facts and norms than by political considerations.

“Everyone should want in an attorney general someone who makes decisions about high-profile cases and the work of the department free from political influence,” said Mr. Kravis, who resigned from the Justice Department in protest over Mr. Barr’s intervention in the sentencing phase of the felony case against Roger J. Stone Jr., a close ally of Mr. Trump.

“I don’t think that the goal should be to replace someone like Bill Barr with the left’s version of Bill Barr,” added Mr. Kravis.

Mr. Biden promised not to direct Mr. Garland on law enforcement actions. The two have spoken only a handful of times, largely about national security matters and policy rollouts, according to an official with knowledge of their encounters. They have also talked during meetings and events at the White House.

Groups that have met with Mr. Garland say that he is attentive, civil and collaborative, even when he rejects their demands. When Al Sharpton and other civil rights advocates recently asked him to help secure passage of voting rights legislation, Mr. Garland demurred. While he has publicly supported the passage of both bills, he told Mr. Sharpton that legislation is for Congress and his job is to enforce laws.

“He stays in the boundaries of where the law will go and where it will not go in terms of addressing matters we bring to him,” Mr. Sharpton said.

But Mr. Garland has been quiet about both his achievements and his contentious decisions. He was more than 100 days into his tenure before he took questions from reporters at a news conference. He declined to discuss with Congress the seizure of data from House Intelligence Committee members. And when asked to explain the department’s thinking on matters like the Carroll case, he stuck with the language in court documents filed by the department.

“He is achieving his principal goal, which is returning the department to a highly professional and nonpartisan institution,” said Neil Eggleston, the former White House counsel under President Barack Obama. “But I would urge him to explain much more clearly some of his controversial decisions that have kept the prior Trump department decisions in place.”

Mr. Garland pays little attention to the daily news cycle but cares deeply that the public understands the Justice Department’s work, according to a senior department official familiar with his thinking.

Mr. Garland and his team may not have appreciated how the media and political environments have changed since he worked in the department in the 1990s, when news coverage mainly came from print and broadcast journalists and before the explosion of social media, said Jamie Gorelick, who was Mr. Garland’s immediate boss when she served as deputy attorney general under President Bill Clinton.

“If you explained your decision to those dozen or so people, that sufficed,” she said. “That is no longer the case. The department has to do more now.”

Former law enforcement officials who worked under presidents of both parties are closely watching how Mr. Garland navigates the department.

“I don’t think an agenda has emerged yet that is obvious,” said George Terwilliger, a top Justice Department official during the George H.W. Bush administration. “The voting case in Georgia raised eyebrows and because it is grounded in the notion that the legislature had a discriminatory intent. That could easily be read as a politicized statement.”

Inside the department, Mr. Garland’s reputation and public vow to respect the recommendations of career prosecutors have helped restore morale, according to career employees who are not authorized to speak publicly about internal matters.

Still, his task is complicated by the lingering scars over how deeply the department waded into politics during the Trump administration and the resulting partisan warfare over the Justice Department.

“Garland is dealing with more than the usual amount of quicksand,” Ms. Gorelick said. “The simple public message is, if Trump would have liked one result, it’s wrong for a Biden Justice Department to decide this way. The complicated message is that the department should follow precedents and what the Justice Department has always done.”