McAleenan previously served as the Customs and Border Protection's commissioner, a position that required a Senate confirmation, and other roles within the DHS umbrella during former Presidents Barack Obama and George W. Bush's tenures.
McAleenan is also proud of having pushed DHS to declare unequivocally that white supremacists pose a growing threat to the American people, a statement the White House previously refused to make in as stark a way, the source close to the acting secretary said.
USA border agents arrested or stopped almost one million undocumented migrants at the southern border with Mexico in the last 12 months, the most in more than a decade, according to statistics released this week.
President Trump tweeted "Congratulations Kevin, on a job well done!"
McAleenan's exit, announced on October 11, comes hours after the State Department announced that, Michael McKinley, a top aide to Secretary of State Mike Pompeo was leaving his post after a 37-year career as a diplomat.
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He took over in April after Secretary Kirstjen Nielsen quit, and he was the fourth person to lead the department in two years. "That's uncomfortable, as the accountable, senior figure".
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.
Earlier this month, the Washington Post described him as increasingly isolated within the Trump administration and overshadowed by others more vocal in their support for President Trump.
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.
A White House official with knowledge of the decision confirmed that it was McAleenan's decision.
McAleenan's exit ends a almost 20-year career in federal law enforcement. "This will only add to the chaos for a Department where there are chronically too many leadership vacancies and positions held by unconfirmed, "acting" officials", he said.
It was unclear whether McAleenan's departure was effective immediately.
DHs' acting deputy secretary, David Pekoske, is next in line to succeed McAleenan.
Still, the acting secretary has been able to point to concrete accomplishments.