He attacked allegations of collusion between the Trump campaign and Russia and wrote a book and produced a documentary during impeachment that were both titled “The Ukraine Hoax.” He has accused former Vice President Joseph R. Biden Jr.’s son of profiting off his father’s name.

Now Michael R. Caputo, a longtime Trump loyalist who made a cameo in the Mueller report, has been installed as the public face of the Health and Human Services Department in the middle of the coronavirus pandemic.

“I’m delighted to have Michael Caputo join our team at @HHSGov as our Assistant Secretary for Public Affairs,” the department’s secretary, Alex M. Azar II, wrote on Twitter on Wednesday with a photograph of the two men already at work together, “especially at this critical time in our nation’s public health history.”

Mr. Caputo, 58, has no background in health care. But what he lacks in expertise, he makes up in loyalty to President Trump, a critical attribute as a superstitious commander in chief has sought to recreate some of the team he assembled four years ago for his stunning upset election victory — and as he has expressed renewed distrust of Mr. Azar.

Mr. Caputo had been passed over for a job early in the administration, according to a person with knowledge of the process, but he contacted administration officials in the past month expressing renewed interest in a position. Mr. Caputo, who is from western New York, has remained friendly with Dan Scavino, another New Yorker and one of the few original campaign aides still on the White House staff. Mr. Scavino played a role in reconnecting Mr. Trump and Mr. Caputo last year, people familiar with the discussion said, which put Mr. Caputo on the president’s radar.

Trump allies heralded the move as in line with the hiring the administration should have done from the beginning: appointing officials devoted to Mr. Trump and his agenda. They described Mr. Caputo as a media-savvy operative who credits himself with good relationships with many journalists.

Critics said a pandemic was no time to rely on hyperpartisans.

“In a crisis, the public is looking for credible, nonpartisan voices with real expertise,” said Dan Pfeiffer, a former White House communications director for President Barack Obama. “Picking a partisan loyalist with a history of promoting conspiracy theories is the exact opposite of the right thing to do but it is very much on brand.”

Mr. Caputo declined to comment for this article, which is based on interviews with six administration officials and others with knowledge of his hiring.

In the decades since Mr. Trump began flirting with running for office, Mr. Caputo has played a role in encouraging him. A protégé of the operative and self-described “dirty trickster” Roger J. Stone Jr., Mr. Caputo has known Mr. Trump since the 1980s, when he briefly worked for a lobbying and political consulting firm started by Mr. Stone and two other Republican operatives, Charlie Black and Paul Manafort. Mr. Manafort went on to run Mr. Trump’s campaign for months and was sentenced last year to more than seven years in prison on a pair of financial and tax fraud convictions.

The firm worked for Mr. Trump in the ’80s and ’90s, both fighting Indian casino expansions that threatened his gambling business in Atlantic City, N.J., and pushing to change the flight path of planes near his Mar-a-Lago club in Florida. At one point, Mr. Caputo even served as Mr. Trump’s driver, delivering him to a congressional hearing on the casino expansions, where Mr. Trump notably testified of some Native Americans running casinos, “They don’t look like Indians to me.”

Mr. Manafort, whom Mr. Caputo regarded with awe, primarily handled that work. Mr. Caputo was more closely allied with Mr. Stone, whom he considered a mentor, and who remained close to Mr. Trump, pushing him to run for president for years before he finally did.

By 2010, Mr. Caputo had resettled in the Buffalo area where he grew up, joined by his second wife, whom he met while working for Mr. Stone in Ukraine advising a parliamentary campaign. He found work that year in local Republican circles, running the campaign for governor of Carl P. Paladino, a wealthy real estate developer who secured the Republican nomination in an upset before losing to Gov. Andrew M. Cuomo nearly 2 to 1 in the general election.

Mr. Caputo then volunteered his service to Mr. Trump in 2014, first advising him as he explored a potential bid for New York governor, and then working with him on an unsuccessful effort to buy the Buffalo Bills of the N.F.L.

Mr. Caputo encouraged Mr. Trump for more than a decade to run either for president or governor of New York, and Mr. Trump drafted him in late 2015 to help run his presidential campaign’s effort in the New York state primary election.

The hiring served as a homecoming of sorts after a peripatetic career that took Mr. Caputo from Washington to Moscow, where he immersed himself in former Soviet culture and politics, met his first wife and worked for American and Russian nonprofit groups and campaigns, advising Russian politicians.

Mr. Caputo also reunited with Mr. Manafort, who asked Mr. Caputo to stay on after the New York primary as a communications adviser working out of the campaign’s Trump Tower headquarters in Manhattan.

But Mr. Caputo was forced to resign in 2016 shortly after celebrating the firing of Mr. Trump’s first campaign manager, Corey Lewandowski, by tweeting a celebratory “Ding Dong the witch is dead.” After leaving the campaign, however, he helped to raise money for a super PAC supporting the campaign, despite having clashed with other Trump advisers.

Mr. Caputo came under scrutiny from Robert S. Mueller III, the special counsel who investigated links between Mr. Trump’s campaign and Russia. A Russian who was directed by a business associate of Mr. Caputo contacted Mr. Caputo in the spring of 2016, saying he wanted to help provide damaging information about Hillary Clinton, according to the Mueller report, citing interviews with both men. Mr. Caputo directed the Russian to Mr. Stone.

Throughout the investigation, Mr. Caputo echoed Mr. Trump’s claims that the inquiry was a politically motivated witch hunt. Mr. Caputo compared his questioning by investigators to a “proctology exam,” and he and Mr. Stone have claimed that the contact with the Russian was a setup by federal law enforcement officials to entrap them. But Mr. Caputo was never charged with a crime, and Mr. Stone, who lost a retrial bid on Thursday, was convicted on charges unrelated to the meeting.

The attention incited threats against him and his family, Mr. Caputo has said, while the legal fees forced him to drain his children’s college tuition fund.

During impeachment, Mr. Caputo again sought to forcefully defend Mr. Trump — as well as Mr. Manafort — by attacking their perceived enemies. In his book, he drew on his experience in Ukraine to try to undermine the case against Mr. Manafort and impeachment, while raising questions about the overlap between Mr. Biden’s diplomacy in Ukraine and his son’s position on the board of a gas company there.

Shortly after the release of the Mueller report, Mr. Caputo and his family traveled to the White House, where Mr. Trump posed for a photo with them in the Oval Office, and, according to Mr. Caputo’s book, dismissed Mr. Biden as “a joke” who was unlikely to win the Democratic presidential nomination.

“Summoned to the White House, talking with the president again, felt like the ending we were hoping for,” Mr. Caputo wrote.

White House officials said that they have been looking for a spot for Mr. Caputo for some time.

“There’s always a pathway back in Trumpworld,” said Jason Miller, a former Trump campaign communications director.

Mr. Caputo’s appointment comes as the president has been looking to install loyalists in senior roles around the administration. He will also serve as a White House ally at a time when Mr. Trump has privately chafed at news reports that he and his top aides believe were planted by Mr. Azar about the administration’s haphazard response to the coronavirus.

Asked for comment, a spokeswoman for Mr. Azar, Caitlin Oakley, pointed to his Twitter post Wednesday night welcoming Mr. Caputo.

Mr. Azar, who several advisers say tried to push Mr. Trump to fight the virus more aggressively, also made missteps when he ran the first coronavirus task force, which was plagued by infighting and oversaw the botched initial efforts to develop widespread testing. Mr. Azar’s status has been imperiled since he persuaded Mr. Trump late last year to agree to restrictions on e-cigarettes, a decision the president later expressed regret about.

“Michael possesses two of the most important attributes to succeed in this administration: The president trusts him, and he’s able to communicate complex policies in a digestible way,” said Kellyanne Conway, an adviser to Mr. Trump.