The meeting between Billy Long and President Trump in the gilded Oval Office last month was supposed to mark a victory for both men.

For Mr. Trump, the ceremonial swearing-in of Mr. Long to his role as the commissioner of the Internal Revenue Service underscored that he at last had a political loyalist and friend, rather than the typical technocrat, at the head of the powerful tax agency.

For Mr. Long, a former Republican congressman from Missouri, it signified the completion of a political comeback. After Mr. Long suffered a dismal showing in Missouri’s 2022 Republican Senate primary, his unwavering and, at times, over-the-top support for Mr. Trump had finally panned out.

As members of his family and close aides from his time in Congress looked on, he was put in charge of one of the federal government’s most visible and fundamental agencies, responsible for collecting roughly $5 trillion in tax revenue each year. By all appearances, Mr. Long had the full support of the president, who had also invited him to a private lunch that day and structured the July 18 swearing-in around it.

But within days, Mr. Long began to lose his grip on a job that he had held for barely a month. Treasury Secretary Scott Bessent had already ousted one White House-installed I.R.S. commissioner this year. He would soon orchestrate the removal of another, culminating in the agency having its seventh leader since January.

At the center of Mr. Bessent’s concerns was the fact that Mr. Long had been trying to put together an independent plan for the I.R.S. without informing the Treasury Department, according to five people familiar with the matter. But Mr. Long’s vision for the I.R.S. had at least in part been shaped by Mr. Trump.

Over their lunch, Mr. Trump suggested people at the I.R.S. whom Mr. Long should fire, while Mr. Long told the president he wanted to elevate a Treasury staff member brought in as part of the Department of Government Efficiency, Sam Corcos, to become his No. 2. Stephen Miller, Mr. Trump’s powerful aide, at one point stopped by the meeting.

Soon after the lunch, the working relationship between Mr. Bessent, a multimillionaire former hedge fund manager, and Mr. Long, a former auctioneer without a college degree, began to break down.

Mr. Long remarked to colleagues, as well as the president, that he had to ask Mr. Bessent for permission for everything he wanted to do at the I.R.S. Mr. Long said he wondered why he had been asked to do the job in the first place, according to people who heard the remark. Treasury officials, after some public gaffes from Mr. Long, began to doubt that he possessed the basic discipline to do the demanding and heavily scrutinized job. Several people in the president’s orbit also questioned whether Mr. Long was up to the task.

Over several weeks, Trump administration officials discussed alternative roles for Mr. Long, and eventually, Sergio Gor, the White House personnel director, informed Mr. Long that he would be moving to a new job, two people briefed on the matter said. Last Friday, Mr. Long said he would leave the I.R.S. and be nominated to become the next ambassador to Iceland.

Mr. Long ultimately lasted less than two months at the I.R.S., the latest in a chaotic stretch at the tax agency. Mr. Bessent has temporarily taken over the position.

In a statement in response to the reporting for this article, a White House spokesman, Harrison Fields, said that Mr. Long was a “steadfast ally” and that the president was “surrounded by highly talented patriots united in advancing his historic and successful agenda.”

A Treasury spokesperson said Mr. Long’s “enthusiasm and ability to connect with people” had helped morale at the I.R.S. “We appreciate his efforts to help kick-start the long overdue modernization of the technical systems within I.R.S. and implement President Trump’s One Big Beautiful Bill,” the spokesperson said.

Mr. Long and the I.R.S. did not respond to requests for comment.

This article is based on interviews with more than a dozen current and former officials at the I.R.S., at the Treasury, at the White House and on Capitol Hill who were granted anonymity to discuss the personnel changes. They described a territorial Treasury secretary in Mr. Bessent who has sought total control of the beleaguered I.R.S., a neophyte administrator in Mr. Long who stumbled during his brief tenure and a rudderless tax agency that is scrambling to execute on its mission amid the leadership turmoil and deep staff cuts this year.

“You say you’re concerned about the debt and the deficit and at the same time you’re creating chaos at the revenue collection agency,” said John Koskinen, who led the I.R.S. during the Obama and first Trump administrations. “This is a complex system.”

‘Upbeat, Friendly and Open’

Mr. Long had almost no background in tax policy or running large organizations when he stepped into the I.R.S. job in mid-June after the Senate confirmed him along party lines. What he lacked in experience he seemed to try to make up for in charisma.

He traveled to I.R.S. offices around the country and held early-morning office hours in Washington, inviting employees to sign up to meet with him individually for 10 minutes. He sent regular, playful missives to the full I.R.S. staff, telling workers about the books on leadership he was reading and repeatedly sending staff members home early on Friday, or “FriYay,” as he called it.

“I used to teach a class on UFOs: ‘Upbeat, Friendly and Open.’ That’s the way I plan to operate, and I hope you’ll join me,” he wrote in his first email to the staff, which was viewed by The New York Times.

For I.R.S. workers, Mr. Long’s attitude was in some ways a welcome change. The period since Mr. Trump took office in January had been marked by upheaval and constant turmoil. In addition to the nearly constant reshuffling of the executive suite, the I.R.S. had also lost roughly 25,000 employees, a quarter of its staff, as the Trump administration razed the ranks of the federal work force. Mr. Long told employees that he did not want to even discuss the possibility of further large-scale staff cuts to the agency.

There were still doubts among the rank and file, though. The only tax work Mr. Long had ever done was pitching small businesses, nonprofits and friends on a pair of tax credits — one riddled with fraud that the I.R.S. had been trying to close down and another that the agency has said does not exist. Under the handle @auctnr1, he posted on social media constantly, regularly amplifying posts from conservative accounts attacking Democrats and even I.R.S. employees. He had abruptly put on leave two senior I.R.S. officials targeted by conservative, anti-tax activists.

His attempts to curry favor with the staff at times caused issues. I.R.S. managers were forced to scramble to schedule the early dismissals. Some employees continued to work overtime as they tried to clear a backlog of international tax returns and correspondence.

“He did not have a very good understanding of the organization or how it operates or the necessary steps to meet the mission,” said Doreen Greenwald, the president of the National Treasury Employees Union. “So, that was concerning early on, but obviously I.R.S. employees care deeply about the success of the organization. They were willing to work with him.”

He also made errors. At a conference of tax professionals in Utah in July, Mr. Long said next year’s tax filing season would start later than normal, a sign of potential disarray at the agency that would come with real repercussions. Such a change would delay the ability of millions to receive their annual tax refunds, a vital source of cash for low-income Americans. He also said the agency’s Direct File program, which allows Americans to file their taxes online with the I.R.S. for free, was dead.

The agency had to walk back both remarks.

A Power Struggle

Mr. Long’s statements in Utah added to frustrations at the Treasury Department, where there were concerns that the commissioner did not properly appreciate that the I.R.S. fell under the umbrella of the Treasury. The I.R.S. is the largest single component of the Treasury Department, making up more than 70 percent of its budget.

But in the second Trump administration, officials from around the government have sought to tap into the tax agency’s vast powers. The Department of Homeland Security has pushed the I.R.S. to share typically confidential taxpayer records it keeps on undocumented immigrants, a process that began last week after months of legal wrangling. Mr. Trump also called for the I.R.S. to strip Harvard of its tax-exempt status, an attempt to use the agency’s auditing powers to put pressure one of the president’s political foils.

In April, Mr. Bessent had to wrest control of the agency back from Elon Musk, who had Mr. Trump install a temporary commissioner at the I.R.S. without consulting the Treasury secretary. Mr. Bessent protested to Mr. Trump, who ultimately acquiesced, saying Mr. Bessent could fire Mr. Musk’s pick, Gary Shapley, after he had led the I.R.S. for just a couple of days. The flap led to a physical confrontation between Mr. Bessent and Mr. Musk in the West Wing, according to multiple people briefed on what took place.

Over his short tenure, Mr. Long made clear that he hoped to put his own mark on the job. He wanted to plan his own events, and in July he attended the National Auction Association’s annual conference, held outside Chicago. Mr. Long, a member of the association’s hall of fame, held a session at the conference, titled “View From the I.R.S.” At one point, he auctioned off a tie that he had signed, with the proceeds going to charity.

“He’s been doing that for as long as I can remember,” said Mike Jones, an auctioneer and friend of Mr. Long’s who said he could not recall how much the tie had sold for.

Mr. Long had at various points floated to Treasury employees the possibility of running the I.R.S. from his hometown in Springfield, Mo., according to people familiar with the remark. Just days before he would step down from the job last week, a phone call Mr. Long had set up with Representative Richard E. Neal, the top Democrat on the Ways and Means Committee, was abruptly rescheduled. An I.R.S. representative had told Mr. Neal’s staff that Mr. Long could not be found in time for the original call.

The I.R.S. is in the middle of trying to put in place the new tax law Republicans passed last month, a process that includes ironing out the final details of Mr. Trump’s campaign promises to not tax overtime or tips, among others. That work is proceeding without Mr. Long, though plans to post a happy birthday message on the internal I.R.S. website for Mr. Long’s 70th birthday on Monday were quietly abandoned.

Mr. Long, for his part, has appeared unbothered by the quick end to his time running the I.R.S., writing on social media that he was “thrilled” to be nominated to become the next ambassador to Iceland. Mr. Long has long celebrated just about everything about Mr. Trump, claiming to have coined the term “Trump train” and distributing fake $45 bills with the president’s face during his first term.

“I was never much of a fighter, but I’m loyal to a fault,” Mr. Long wrote to I.R.S. employees on July 25. “Loyal to my family, my employees partners and my industry. I will stand up and fight for those I’m loyal to.”

Alan Rappeport contributed reporting from Washington.