Opponents of the process call it "chain migration", but Melania Trump may have used the policy to sponsor her parents.
Viktor and Amalija Knavs's lawyer Michael Wildes said the Slovenian couple took the citizenship oath on Thursday in New York City.
While the Knavs now live in Mar-a-Lago and Trump Tower in New York City, they didn't always live in luxury.
It is believed that Melania Trump, who is a USA citizen, has sponsored her parents for permanent residency.
Asked about President Trump's heavy rebuke of family-based migration, Wildes said: "I can't comment on the President's politics when it comes to my clients but I have stood up against the President's immigration policies personally".
The first lady's office declined to comment.
President Donald Trump's in-laws are officially United States citizens, obtaining their citizenship through the sponsorship of their adult daughter, one of the very categories of family visas that the administration has sought to end.
Trump also often speaks out against the family-based immigration system at his rallies, saying it a pathway for terrorists to enter the country, the NYT report added. "This is an example of it going right", he said.
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Their son-in-law has frequently attacked USA immigration laws, calling them the "dumbest laws on immigration in the world".
The Republican president argues that the system steals jobs from Americans and threatens national security, calling for a merit-based system that preferences more educated, English-speaking professionals. The Knavses came and went from the ceremony at a Manhattan federal building flanked by Homeland Security police.
Kaitlyn Schallhorn is a Reporter for Fox News.
Viktor, 74, and Amalija, 73, had been living as permanent residents in the US.
The Knavses' citizenship ceremony was kept hush-hush around the Jacob K Javits Federal Building. The Slovenian immigrants, a former auto dealer and textile factory worker, had been living in the U.S.as permanent residents.
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The couple met in Sevnica, Slovenia in 1966, and Melania was born in the same town.