In 2016, Thomas D. Homan was a frustrated immigration bureaucrat ready to call it quits.

A former border patrol agent with a lawman’s steely demeanor, he had been an odd fit for the Obama administration. Top officials would call on him when they wanted a hard-liner’s take. But his proposals — including an early version of the controversial family separation policy to deter migrants — were often rejected.

In the years since, Donald J. Trump’s rise has fully unleashed Mr. Homan and his ideas. In Mr. Trump’s first administration, Mr. Homan helped make the family separation policy a chaotic reality. And in the second, Mr. Homan is poised to be the White House “border czar,” tasked with making good on the incoming president’s promise to carry out the largest deportation campaign in American history.

Mr. Homan’s ascent completes his transformation from dutiful official in a Democratic administration to full-throated Trump world fixture.

Where he once signed off on transgender care guidelines and the Obama administration’s targeted immigration policy, he now endorses far-right theories about immigration and elections. He has boasted that he is completely unbothered by criticism of the family separation policy.

When he was first approached about joining a second Trump administration, Mr. Homan says he told Mr. Trump that he was so angry about the border that “I’ll come back for free.”

Mr. Homan, whose post does not require Senate confirmation, has acknowledged his mission is virtually impossible, much less in four years. Mr. Trump has vowed to remove all of the roughly 11 million people in the country illegally — which experts say is far-fetched and would be shockingly disruptive if carried out.