James W. Loewen, a sociologist and civil rights champion who took high school teachers and textbook publishers to task for distorting American history, particularly the struggle of Black people in the South, by oversimplifying their experience and omitting the ugly parts, died on Thursday in Bethesda, Md. He was 79.

His death, in a hospital, was confirmed by Ellen Adler, his publisher at the New Press, who said he died after an unspecified “long illness.”

“Those who don’t remember the past are condemned to repeat the 11th grade,” Dr. Loewen wrote in “Lies My Teacher Told Me: Everything Your American History Textbook Got Wrong” (1995), the best known of his dozen books attacking historical misconceptions.

Dr. Loewen was a relentless contrarian who challenged anyone who imagined academic life as a passage through genteel lectures on settled matters for drowsy students on leafy campuses. He charged through history like a warrior, dismantling fictions and exposing towns for excluding minorities; teachers and historians for dumbing lessons down; and defendants in 50 class-action lawsuits who, according to his expert testimony, victimized people in civil rights, voting rights and job discrimination cases.

A Northerner fascinated with Mississippi, he wrote his first book about the Chinese population there. He wrote another about how America’s historic sites distort our knowledge of the past. And it was a mistake to get him started on the origin of Thanksgiving: Plymouth was already a village with cleared fields when the Pilgrims found it deserted by plague victims. No turkey was served in 1621 — perhaps it was duck. And there was no pie. The settlers had no wheat flour for crust and no oven for baking. The holiday Americans celebrate has nothing to do with the Pilgrims. It was invented 242 years later by Abraham Lincoln to celebrate the North’s victory at Gettysburg.

“History is by far our worst-taught subject in high school,” Dr. Loewen told The Atlantic in 2018. “I think we’re stupider in thinking about the past than we are, say, in thinking about Shakespeare, or algebra, or other subjects. Historians tend to make everything so nuanced that the idea of truth almost disappears.”

The son of a doctor and a librarian, Dr. Loewen was raised in Illinois and educated in Minnesota and at Harvard, and he began his half-century as a university professor of sociology in 1968 at Tougaloo College, a historically Black liberal arts institution in Mississippi. Facing his first freshman class, he posed a seemingly simple question for 17 students: “What is Reconstruction?”

“Well,” he recalled them saying, “Reconstruction was the period right after the Civil War when Blacks took over the government of the Southern states. But they were too soon out of slavery, and so they screwed up and white folks had to take control again.”

Dr. Loewen’s heart sank, he told NPR in 2018. It was a glimpse of the enormous task before him: setting the historical record straight for Mississippi’s — and America’s — young students.

Patiently, he explained to his class: Black people never took over the Southern states. They all had white governors, and all but one had white legislative majorities. Reconstruction governments did not “screw up.” They created the best constitutions the South had ever had, and better governments than any others in the South in the 19th century. And whites did not put things right by taking control again. The people who took charge were white supremacists, and some were original Ku Klux Klansmen.

As to the problem of textbook distortions, Dr. Loewen found that state review and purchasing panels controlled the use of public schools’ books. The history text widely in use for years in Mississippi described Black people as complacent or troublemaking. It said Black officeholders during Reconstruction were corrupt, and it called the Ku Klux Klan a “secret social and fraternal club.” Lynching was not even mentioned.

Dr. Loewen and the historian Charles Sallis of Millsaps College crafted an extraordinary response over several years, co-writing and editing “Mississippi: Conflict and Change” (1974), a total revision of the state’s historical past. With an interdisciplinary approach that was less concerned with linear facts and dates, the book examined the social, political and cultural components of Mississippi life throughout history.

It profiled politicians, blues singers, writers and others who left their mark in different fields. It detailed years of slavery, the Civil War, Reconstruction and the modern struggle for civil rights, the Supreme Court ruling against school segregation, and current accounts of race relations and conflicts. It called the K.K.K. a terrorist organization created to preserve a “Southern way of life,” and said that Black schoolchildren had been kept segregated under bogus “separate but equal” doctrines.

Pantheon Books published “Mississippi: Conflict and Change,” which won the 1976 Lillian Smith Book Award for best Southern nonfiction. But Mississippi officials vetoed its use in schools, calling it racially inflammatory. The NAACP Legal Defense and Education Fund and the Mississippi-based Lawyers Committee for Civil Rights Under Law sued in federal court on behalf of Dr. Loewen and his co-authors.

In 1980, a United States District Court, citing First and Fourteenth Amendment freedoms, ruled in favor of Dr. Loewen and his colleagues. The American Library Association called it a victory for the “right to read freely.”

Acceptance of “Conflict and Change,” and its prompt use by 26 of 150 school districts in the state, began what Dr. Loewen called a sea change in Mississippi history books. The book remained in use there for six years. And in succeeding years, many authors wrote, and the state accepted, more objective and comprehensive volumes.

There were setbacks, though. Surveys still show some districts and individual teachers continuing to use outdated or inaccurate history textbooks.

Dr. Loewen left Tougaloo College in 1975 and for most of the next 20 years taught sociology, with an emphasis on race relations, at the University of Vermont. In 1995 he published “Lies My Teacher Told Me,” his study of 12 history textbooks widely used in America. That book, which accused historians of propagating blind patriotism and sanitized optimism, was acclaimed by critics and won the American Book Award. Updated editions were issued in 2005, 2008 and 2018 by the New Press, which has called the book its all-time best seller, accounting for the bulk of almost two million Loewen books sold.

“Jim Loewen’s great achievement was his ability to combine meticulous, dogged research with humor and messianic zeal to correct the way history is taught in textbooks — which is to say all too often with large doses of xenophobia, racism, sexism and outright lies,” Ms. Adler of the New Press said in an interview.

James William Loewen was born on Feb. 6, 1942, in Decatur, Ill., to David and Winifred (Gore) Loewen. He and his sister, Mary, grew up in Decatur, where he was a National Merit Scholar and graduated from MacArthur High School in 1960.

He married Patricia Hanrahan in 1968; they divorced in 1975. In 2006, he married Susan Robertson. She survives him, as do his children from his first marriage, Nicholas Loewen and Lucy McMurrer; his sister, Mary Cavalier; and four grandchildren.

He attended Carleton College in Northfield, Minn., where he began to question what he had learned in his high school history classes. He earned a bachelor’s degree in 1964. At Harvard, he received a master’s degree in 1967 and a doctorate in 1968.

His research led to his first book, “The Mississippi Chinese: Between Black and White” (1971), about 19th-century immigrants lured to the state by promises of work, and their descendants, many of whom became grocers.

Since 1996, Dr. Loewen had lived in Washington, where he was a visiting professor at the Catholic University of America. He lectured widely and wrote other books, including “Lies My Teacher Told Me About Christopher Columbus: What Your History Books Got Wrong” (1992) and “Lies Across America: What Our Historic Sites Get Wrong” (1999).

His book “Sundown Towns: A Hidden Dimension of American Racism” (2005) documented the stories of thousands of communities from 1890 to 1968 that systematically, and often forcibly, excluded Black people, Jews and others. The word “sundown” referred to signs at city limits that warned Black people not to “let the sun go down on you” there.