Two weeks after Johnnie A. Jones Sr. graduated from law school in 1953, he was thrust into a case that would set a template for the civil rights movement, and for his own legal career: He was recruited to help represent people who had been arrested during a bus boycott in Baton Rouge, the Louisiana capital.

Lasting eight days, it was the first large-scale bus boycott of the civil rights era. And it served as a model for other nonviolent resistance protests, especially the more famous yearlong bus boycott that began in December 1955 in Montgomery, Ala., spurred by the arrest of Rosa Parks. The Montgomery organizers, led by a charismatic young preacher named the Rev. Dr. Martin Luther King Jr., consulted with Mr. Jones and others on tactics and strategy.

The Baton Rouge boycott also marked the beginning of Mr. Jones’s 57-year career as a persistent challenger to the race-based codes of the Jim Crow South. He was the first Black member of the Baton Rouge Bar Association.

Mr. Jones was 102 when he died on April 23. A goddaughter, Mada McDonald, told WAFB-TV in Baton Rouge that he had died at the Louisiana War Veterans Home in Jackson, La.

In addition to his civil rights history, Mr. Jones had a brush with military history. During World War II he was the first Black warrant officer in the Army. And he participated in Operation Overlord, in which Allied forces landed more than 150,000 troops on Normandy beaches in 1944 as part of the largest amphibious assault in the history of warfare.

As for his career as a litigator, Mr. Jones became involved in numerous civil rights cases, often working with the N.A.A.C.P. and the Congress of Racial Equality. He sought to remove racial identification from election ballots and fought to integrate Baton Rouge’s schools, parks and pools, all the while facing threats of arrest and disbarment; bombs were twice planted under his car.

After the United States Supreme Court outlawed segregation in public schools in the landmark 1954 decision Brown v. Board of Education, Mr. Jones still had to accompany Black children to school for their own protection, he said.

He also defended several students from Southern University, the historically Black institution in Baton Rouge, after they staged nonviolent lunch-counter sit-ins in the city but were arrested anyway and charged with disturbing the peace. By the time the sit-in cases reached the Supreme Court in 1961, they were being argued, successfully, by Thurgood Marshall, then a young civil rights lawyer who later became the first Black justice of the Supreme Court.

Johnnie Anderson Jones was born on Nov. 30, 1919, in Laurel Hill, a tiny town in northern Louisiana, and raised on a plantation, where his parents, Henry Edward and Sarah Ann (Coates) Jones, were farmers on 75 acres of rented land.

After he enrolled at Southern University, Mr. Jones was drafted into the Army in 1942 and assigned to a unit responsible for unloading equipment and supplies on Omaha Beach during the D-Day invasion.

He was almost killed twice, the first time when a mine exploded below his ship, blowing him onto an upper deck. Then, as he waded ashore as part of the Allied assault, he came under fire from a German sniper. Before the war was over, he had fought in the Battle of the Bulge.

While most of the soldiers on D-Day were white, roughly 2,000 of them were Black service members. By the end of the war, more than a million African Americans were in uniform, including the famed Tuskegee Airmen. But the military was still segregated by race, and these soldiers encountered discrimination both in the service and when they came home.

When he was honorably discharged from the Army, Mr. Jones was described as white, he recalled in an oral history in 1993. He said the clerks filling out his papers had assumed he was white because they didn’t think a Black person could have performed the tasks that he was listed as having performed.

“Right now I’m white, as far as my discharge paper, because I didn’t go back to have it corrected,” he said, laughing at the recollection.

Back in Louisiana, by his account, he was driving to a medical appointment in New Orleans one day, to have wartime shrapnel removed from his neck, when he was pulled over and beaten by a white police officer.

“He knocked me down and started kicking me,” Mr. Jones told the Department of Veterans Affairs in a 2021 interview. The incident helped compel him to become a lawyer, he said.

“Things weren’t right,” he said. “‘Separate but equal’ was unconstitutional, and I wanted to fight it and make it better.”

Mr. Jones resumed his college studies at Southern and earned his bachelor’s degree in psychology in 1949. He worked for the Postal Service as a letter carrier, then earned his law degree from Southern University School of Law (now Southern University Law Center). He was asked to head the civil rights division of the Department of Justice by Attorney General Robert F. Kennedy, he said, but the appointment never materialized in the wake of President John F. Kennedy’s assassination shortly thereafter.

Mr. Jones continued to practice law into his 90s.

His marriage, to Sebell Chase, ended in divorce. His four children and his seven siblings all died before he did. He is survived by numerous grandchildren and great-grandchildren.

Only last year, 77 years after being wounded during the war, Mr. Jones was belatedly awarded the Purple Heart at the Old State Capitol in Baton Rouge.

“I want to express our deepest respect for your distinguished service, and long overdue recognition of your wounds received during the invasion of Omaha Beach on D-Day,” Gen. James C. McConville, the Army chief of staff, wrote in a letter to Mr. Jones accompanying the award.

“We owe you a debt of gratitude,” he added, “both for your sacrifices during World War II and for being a role model for African Americans aspiring to serve.”