John Doar, a country lawyer from northern Wisconsin who led the federal government’s on-the-ground efforts to dismantle segregation in the South, and who later headed the team that made the case for impeaching President Richard M. Nixon in the wake of the Watergate scandal, died on Tuesday at his home in Manhattan. He was 92. The cause was congestive heart failure, his son Robert said.

During the most volatile period of the civil rights movement in the 1960s, it was Mr. Doar, along with a federal marshal, who escorted James Meredith when he integrated the University of Mississippi in 1962. It was Mr. Doar who led the successful prosecution of the men who killed three young civil rights workers in Mississippi in 1964. And it was Mr. Doar who defused a dramatic standoff between bottle-throwing civil rights protesters and police officers with their guns drawn in Jackson, Miss.

“He was the face of the Justice Department in the South,” President Obama said in 2012 when he presented Mr. Doar with the Presidential Medal of Freedom, the country’s highest civilian honor. “He was proof that the federal government was listening.”

In 1974, Mr. Doar, a Republican, was named by Democrats to be chief counsel to the House Judiciary Committee investigating Watergate.