As a district court judge on Long Island, Joseph Nocella Jr. presided over low-level criminal cases and disputes involving small amounts of money. He could soon oversee some of the country’s highest-profile criminal cases.

This week, President-elect Donald J. Trump announced that Mr. Nocella was his pick to be the next U.S. attorney for the Eastern District of New York. In a statement announcing his choice on Truth Social, Mr. Trump said Mr. Nocella had “a strong record of bringing Law and Order to the incredible people of New York.”

As a Long Island judge, Mr. Nocella has adjudicated matters like Covid-19 relief fraud and drunken-driving cases and presided over family court.

The Eastern District, where Mr. Nocella was a prosecutor decades ago, operates at a different scale: The office and its 160-some prosecutors have long pursued high-profile cases involving organized crime, national security and foreign politicians. It has won convictions in a variety of recent blockbuster cases, including against the singer R. Kelly, who was convicted of sex crimes; the disgraced former congressman George Santos; and leaders of the sex cult NXIVM.

Seth DuCharme, who served as acting U.S. attorney in 2020, said Mr. Nocella’s range of experiences, including presiding over a small court, would serve him well in the district, which he called “a friendlier version” of its more high-profile neighbor, the Southern District of New York. But he noted that Mr. Nocella was something of an outsider.

“I think that probably most of the prosecutors were surprised,” Mr. DuCharme said. “It’s not a name they’ve heard about a lot.”