Dorothy Seymour Mills, who helped her husband, Harold Seymour, compile the first in-depth historical studies of baseball but who did not receive full credit for her research, editing and writing until after his death, died Nov. 17 at a hospital in Tucson. She was 91.

The cause was complications from an ulcer, said Jacob Pomrenke, director of editorial content of the Society for American Baseball Research.

In the 1940s, Harold Seymour was working on his doctoral dissertation in history at Cornell University, working on the 19th-century origins of baseball. At the time, he was teaching in Cleveland at Fenn College, which later merged with Cleveland State University. Ms. Mills was one of his students.

They struck up a relationship and, after his divorce, were married in 1949. She helped with his research as he received his doctorate in 1956, then revised his dissertation for a book, “Baseball: The Early Years,” published in 1960. It was the first time the subject had been addressed by a trained academic historian and became a landmark in the study of baseball.

A second volume, “Baseball: The Golden Age,” appeared in 1971, followed by “Baseball: The People’s Game,” in 1990. Seymour was lauded as a groundbreaking historian who examined baseball as a business enterprise and cultural phenomenon of far-reaching social importance.

“For three decades Harold Seymour has been not merely the most respected of the game’s historians but also the standard-setter against whose work all others have been weighed,” Washington Post book critic Jonathan Yardley wrote in 1990. “Seymour accomplished what no one before him had: He legitimized baseball as a subject for serious historical inquiry, and in the process he accumulated more information about the game than had been available in any other published histories.”

When Seymour died in 1992, his ashes were scattered at Doubleday Field in Cooperstown, N.Y., the town where baseball was supposedly created in 1839. (It had been played in various forms since the 18th century, including by George Washington’s soldiers at Valley Forge.)

The myth of baseball origins was one of many shattered by Seymour’s historical research.

Another myth was that he worked alone.

Seymour praised his wife, then known as Dorothy Z. Seymour, for her “indispensable” work in his acknowledgments, but her contributions turned out to be much greater than most people realized.

Several years before Harold Seymour’s final book, “The People’s Game,” appeared in 1990, he was showing the clear effects of Alzheimer’s disease. Ms. Mills, who had done much of the research and editing for her husband for more than 40 years, ended up writing 13 of the book’s 37 chapters, according to a formal claim she submitted to her husband in 1989.

“I told Harold that my name should be on the books as a contributor,” she told the Naples (Fla.) Daily News in 2008.

“ ‘I just can’t do that,’ ” she said her husband told her. “I think it was his Alzheimer’s that made him say that.”

In 2004, Ms. Mills published a memoir, “A Woman’s Work: Writing Baseball History With Harold Seymour,” in which she detailed the partnership — “at once glorious and ignominious” — that made her husband the renowned dean of baseball historians but left her unrecognized.

“He boldly opened the field of baseball as a subject for serious study,” she wrote, before adding: “My star has always been outshone by Seymour’s, and I permitted this to happen. I even believed, and told him, that ‘your work is more important than mine.’ That was like saying, ‘You are more important than I am.’ ”

She noted that her husband did not know how to type and disliked the drudgery of research. She did much of the initial legwork in libraries around the country, compiling thousands of note cards and writing outlines that often found their way, in her original wording, into the books.

She typed and edited her husband’s manuscripts, obtained photographs and performed the laborious work of correspondence, indexing and preparing bibliographical material.

“Everyone assumed that he had done all that work by himself — that’s what he wanted them to assume, but we were equal partners,” Ms. Mills told the New York Times in 2010. “All these things were done jointly. He just couldn’t share credit. And I didn’t say anything at the time, because at the time, wives just didn’t do that.”

Harold Seymour, who was 18 years older than his wife, was an often prickly and gruff character who insisted on being addressed as “Dr.”

“He thought himself very important but insufficiently recognized,” Ms. Mills told CNN in 2011. “He was often more ready to deliver insults than to give praise.”

In 1993, the Society for American Baseball Research, or SABR, included Ms. Mills’s profile alongside that of her husband on an annual medal awarded to the author of the year’s best book of baseball history.

But in 2010, when SABR posthumously named Harold Seymour as one of the inaugural recipients of the Henry Chadwick Award for contributions to baseball history, it did not extend the honor to his widow. Fierce arguments broke out when it appeared that Ms. Mills was being slighted once again. The SABR members quickly altered their decision, giving equal credit to Ms. Mills.

The next year, Oxford University Press agreed to include her name as co-author on all three of the baseball histories written with her husband.

“Their work was brilliant and original in its focus on off-the-field activity,” John Thorn, the official historian of Major League Baseball, wrote in a message to The Post. “If someone were to ask me, what should be the first baseball book I should read to understand the history of the game, I would point them to the Seymours.”

Dorothy Jane Zander was born July 5, 1928, in Cleveland. Her father was a printer, her mother a homemaker.

She had early ambitions of being a journalist. After she and Seymour were married, she then transferred to what is now Case Western Reserve University, from which she graduated.

They lived in New York and Massachusetts, where her husband taught in colleges and Ms. Mills taught in elementary schools. She later worked for a Boston publishing company and wrote several children’s books, while helping with her husband’s baseball research.

Harold Seymour gave up teaching in 1969, and the couple, who had no children, moved to Ireland and later to Alabama, South Carolina and Massachusetts before settling in New Hampshire.

After her husband’s death, she married Roy Mills, a onetime officer in the Royal Canadian Air Force, and settled in Naples, Fla. He died in 2012. Survivors include four stepchildren and six grandchildren.

Ms. Mills, who also wrote under the name Dorothy Jane Mills in later years, published about 30 books in all. She was a skilled pianist who often played “Take Me Out to the Ballgame” at SABR conventions. In 2017, SABR named an award in her honor for significant contributions to women’s role in baseball.

In 2010, revealing what could be called a dirty little secret, she said, “I’m still not a fan of baseball. People can’t understand that. I think it’s a good idea to remain above that. You write a lot more objectively about a subject you’re not in love with.”