Acting Homeland Security Secretary Kevin McAleenan has resigned after spending six months on the job, according to a tweet Friday night from President Donald Trump.
"Kevin now, after many years in Government, wants to spend more time with his family and go to the private sector," Trump said.
McAleenan becomes the latest top-level Trump adviser this year to step down amid reports of frustrations with the job and clashes with other administration officials.
"I want to thank the President for the opportunity to serve alongside the men and women of the Department of Homeland Security," he said in statement after the announcement. "With his support, over the last 6 months, we have made tremendous progress."
A career law enforcement official who served in the Bush and Obama administrations at U.S. Customs and Border Protection, McAleenan was tapped by Trump to orchestrate the U.S. response to a massive influx of undocumented migrants at the U.S.-Mexico border.
He was confirmed as head of CBP in March 2018, then named "acting" DHS chief in April after Kirstjen Nielsen was pushed out and Trump declared he wanted to take the nation's immigration policies in a "tougher" direction.
Trump never officially nominated McAleenan, keeping him in an "acting" role, which is now commonplace for a president who said he prefers the "flexibility."
A person familiar with his thinking said McAleenan had considered returning to the private sector some two years ago and never expected last April to be tapped by Trump to take over the Homeland Security Department, a sprawling agency created in the wake of 9/11 tasked with preventing future terror attacks, enforcing the nation's immigration laws and securing elections.
The person, who spoke on condition of anonymity in order to discuss sensitive personnel issues, said the acting secretary felt he had accomplished two of his top priorities -- reducing the number of undocumented border crossings and identifying "violent white supremacy" as a threat to the nation in a key U.S. strategy document.
After a decade-long sprint in government service, the person said, McAleenan wanted to spend more time with family, including his two young children.
McAleenan's resignation comes amid reports that he also felt undermined by immigration hardliners in the administration who frequently appeared on conservative news outlets calling for tougher policies.
Three officials in particular -- Mark Morgan, acting chief of U.S. Customs and Border Protection, Ken Cuccinelli, acting director of U.S. Citizenship and Immigration Services and Matthew Albence, acting head of U.S. Immigration and Customs Enforcement -- have aggressively advocated for stepped-up enforcement, including large-scale deportation raids targeting families.
Last June, a close Trump ally -- Brandon Judd, head of the National Border Patrol Council -- accused McAleenan of trying to undermine Trump's efforts to curb illegal immigration and insisted he was too liberal to effectively manage immigration enforcement.
McAleenan, at least publicly, has mostly declined to discuss internal politics. But in a recent interview with The Washington Post, he lamented that the public "message" on immigration had grown so polarizing.
"I think the words matter a lot," McAleenan told The Post. "If you alienate half of your audience by your use of your terminology, it's going to hamper your ability to ever win an argument."
While McAleenan is widely seen in government circles as more apolitical than many Trump loyalists -- in 2015, he won the highest civilian service award -- his time as acting secretary put him front and center as point man on some of the president's most controversial immigration policies.
Shortly after being confirmed as CBP chief, McAleenan helped to enforce Trump's zero-tolerance border policy, which resulted in some 2,600 children being separated from their parents in a matter of weeks so the adults could be detained elsewhere.
Internal investigators later determined the policy was poorly managed and noted the government lacked a central database to quickly reunite the families. One government report documented the emotional devastation on children in harrowing terms, describing kids who couldn't stop crying and who struggled to breathe because of the stress. One boy, according to the report, assumed his father has been killed and that he would be next.
McAleenan has since called the policy a mistake because, he said, it lost the public trust and was "not worth it."
But he continued to embrace other tough anti-immigration measures pushed by the administration. McAleenan was among several top officials who publicly backed Trump this year when he threatened tariffs against Mexico unless its government did more to stop the migrants. And he dramatically expanded a policy initiated under Nielsen that forced migrant families to wait in Mexico while their asylum claims were heard.
Human rights groups say the "Remain in Mexico" policy has created a severe humanitarian crisis in northern Mexico towns, where food and jobs are scarce and kidnappings and crime are increasing.
McAleenan also has denied allegations of mistreatment and neglect of children at U.S. Border Patrol facilities, even as the DHS inspector general documented in an internal investigation massive overcrowding and unsanitary conditions.
These policies have made McAleenan a target on the left. On Monday, progressive protesters prevented him from speaking at Georgetown University's law school, eventually forcing him from the stage.
From McAleenan's perspective, U.S. laws limiting the detention of children have only encouraged families to travel with kids to the border. And the result of the tougher policies this year is a much-welcome drop in border crossings, from some 144,000 last May to 52,000 in September.
"Despite the obvious dangers of the journey, smugglers have adapted their craft to exploit the weaknesses in our immigration system," he said, in remarks prepared for the Georgetown speech he wasn't allowed to give but later released. "Their operations are highly sophisticated -- with calculated planning on when and where to cross our borders."
McAleenan's decision to step down comes as the president faces an impeachment inquiry for his handling of discussions with the Ukraine government. McAleenan is not tied to that inquiry, spending the vast majority of his career in government has been focused on border security.
When asked whether he ever stood up to Trump, McAleenan told ABC News last May that he did and his relationship with the president was better because of it.
"I'm a person of integrity," McAleenan said. "I've been a career law enforcement officer. I think [Trump] expects no less than the best facts and the best recommendations I can make to him, and I'll continue to do that."
McAleenan was nearing the deadline for serving in an acting role. The Federal Vacancies Reform Act allows acting officials to service 210 days without a formal nomination, possibly longer pending a confirmation.
It was not immediately clear who would replace him. Trump tweeted that he "will be announcing the new Acting Secretary next week. Many wonderful candidates!"
Last April, McAleenan picked David Pekoske, the Transportation Security Administration administrator, to work as his deputy.
ABC News' Luke Barr contributed to this report.