James Nabrit, a civil rights lawyer who fought school segregation before the Supreme Court and helped ensure that the Rev. Dr. Martin Luther King Jr.’s 1965 march from Selma to Montgomery, Ala., was allowed to go forward, died Friday in Bethesda, Md. He was 80.

The cause was lung cancer, said Ted Shaw, a close friend and the former director-counsel at the NAACP Legal Defense and Educational Fund.

Nabrit, who worked at the defense fund from 1959 to 1989, argued 12 cases before the Supreme Court and won nine. For many years he served as the low-profile but essential second-in-charge when the group was the most persistent and prominent legal voice fighting to enforce school integration and end Jim Crow laws in the South.

“Jim was involved in many of the most important matters of the civil rights movement,” Shaw said. “The public didn’t know who he was, but civil rights lawyers knew him.”

Nabrit grew up among pillars of the civil rights movement. His father, James Nabrit Jr., helped Thurgood Marshall argue the cases that led to the Supreme Court’s landmark Brown v. Board of Education decision in 1954 and later became president of Howard University in Washington.

The younger Nabrit also worked with Marshall, who hired him as a lawyer with the NAACP Legal Defense and Educational Fund in 1959. Marshall, who later became the first black Supreme Court justice, founded the fund in 1957.

“When I was hired, he announced to everyone that my job title was ‘low man on the totem pole’ and that I was to be addressed as ‘boy,’” Nabrit recalled in a 2001 interview with the magazine The Washington Lawyer. “He always kept everyone laughing.”

Nabrit’s first assignment was to help write a Supreme Court brief arguing against an appeal of a decision that Marshall had won in Louisiana. The lower court ruling was affirmed.

In 1965, Nabrit helped write a comprehensive plan for a 50-mile march for voting rights from Selma to Montgomery that the Alabama authorities were trying to prevent. It was written to help bolster a claim by King and his associates that they had a constitutional right to conduct the march.

After a judge approved the plan that Nabrit had helped write, the march — which eventually included 25,000 people — went forward later that month. Congress passed the Voting Rights Act that summer.