In five years of pro baseball, Mary Pratt threw a no-hitter, notched a 20-win season and helped lead her team to the league championship series, all while wearing makeup and above-the-knee skirts. Like everyone else in the All-American Girls Professional Baseball League, she was expected to follow a simple rule: “Look like women. Play like men.”

Ms. Pratt, who died May 6 at 101, was one of the first members of the Rockford Peaches, a powerhouse Illinois team formed in 1943 and immortalized in director Penny Marshall’s sports comedy “A League of Their Own.”

Although the Peaches and the All-American league folded after a dozen seasons, Ms. Pratt helped keep their legacies alive, appearing at baseball conventions and other events to champion women in sports.

“As some of the other former players will tell you, once she was on the stage you couldn’t get her off,” said Ted Spencer, former chief curator of the National Baseball Hall of Fame in Cooperstown, N.Y. “She was very dedicated to telling the story that young women and girls deserve an equal chance in all sports, not just baseball.”

Growing up in Connecticut and Massachusetts, decades before Title IX legislation spurred the growth of women’s athletics, Ms. Pratt played “anything that the boys would let me join.” She shot hoops through a neighbor’s peach basket, played catch at the park and ultimately pitched for a Boston softball team known as the Olympets while in college, once traveling to New York to play at Madison Square Garden.

Ms. Pratt was 24, teaching physical education in suburban Quincy, Mass., when she joined the Rockford Peaches on a $60-a-week contract during the All-American’s first season. Although women had played baseball since the game was invented — trouser-clad “bloomer girls” competed against men’s teams beginning in the late 19th century — the All-American marked a new pro showcase for female athletes.

“The league was a forerunner for women’s professional team sports in the United States,” said Carol Sheldon, a board member of the AAGPBL Players Association, a nonprofit organization that promotes the league’s history.

In a phone interview, she recalled that the league began as a hybrid of softball and baseball, founded as the All-American Girls Softball League by Philip K. Wrigley, the Chicago Cubs owner and chewing gum magnate. In part, it was created to fill stadiums and entertain Midwestern cities during World War II, when male players were being drafted into the military.

Rule changes effectively made the league a women’s version of Major League Baseball, as base paths were lengthened, the ball was reduced in size and the underhand pitching motion was replaced by sidearm and eventually overhand techniques. By the league’s peak in the late 1940s, nearly 1 million fans were going to All-American games each year.

Ms. Pratt, a slight left-hander at 5-foot-1 and 125 pounds, acknowledged that she was far from a great hitter, with a career batting average of .144. But her slingshot throwing motion made her a reliable and often dangerous force on the mound: In 1944, her best season as a pitcher, she went 21-15 and had an earned-run average of 2.50.

As a result of injuries and league policies aimed at promoting competition, she was reassigned early that season to the Kenosha Comets, a Wisconsin squad led by manager Marty McManus, a former MLB player and Boston Red Sox skipper. “Prattie,” as she was known, went on to throw her no-hitter and help lead the Comets to a championship series against the Milwaukee Chicks, which Kenosha lost in seven games.

Ms. Pratt played one more season with the Comets but spent the rest of her career with the Peaches, compiling a win-loss record of 28-51 and an ERA of 3.48. “I had one good year, according to statistics,” she later told the Society for American Baseball Research, “but five wonderful years in a project of its nature which has never been duplicated during my years of competitive athletics for girls and females.”

“The Belles of the Ballgame,” as Ms. Pratt and her fellow athletes were sometimes known, faced strictures that extended far beyond the baseball diamond. They were required to keep their hair in either short curls or at shoulder length, ordered to attend charm-school classes overseen by cosmetics mogul Helena Rubinstein, and forbidden from smoking or drinking. Chaperones traveled with the teams to make sure they followed the rules.

“We were going to look like ladies, dress like ladies and act like ladies. I lived the life,” Ms. Pratt told the Associated Press in 2012. Still, she said, “the Queens of Swat” were far from swaddled. Because of the skirts they wore, Ms. Pratt and other women often got scrapes known as “strawberries” from repeatedly sliding during an approximately 125-game season.

“I shouldn’t do it, but sometimes I look today and see how the boys are treated well,” she said in a 2009 oral history, referring to MLB players. “They can’t pitch nine innings. And to think that we had our strawberries and we were playing every night, so we must have got a few aches and pains.” Not that it mattered, she added: “I think everybody will tell you that we were having so much fun.”

Ms. Pratt was born in Bridgeport, Conn., on Nov. 30, 1918, in the midst of the flu pandemic and with national women’s suffrage still on the horizon. Her father was a draftsman, later an accountant, who moved the family to his hometown of Quincy during the Great Depression.

In 1940, Ms. Pratt received a bachelor’s degree in physical education from Boston University. She went on to teach for 46 years, leading gym classes in addition to coaching or officiating basketball, softball, field hockey and lacrosse.

When the head coach position opened up at North Quincy High’s football team, she made headlines by applying. And when Title IX was signed into law in 1972, she became involved in local and state athletic associations to promote girls’ sports, later co-founding the organization New Agenda-Northeast to further her efforts.

“I admired my opportunity to play in the All-American league, but I want to be remembered as a schoolteacher,” she told Jim Sargent for the book “We Were the All-American Girls.”

It was partly through one of her former students — Spencer, the National Baseball Hall of Fame curator — that the All-American league was rediscovered. After closing down in 1954 amid a decline in popularity, the league was all but forgotten until 1988, when Spencer launched a “Women in Baseball” exhibit at the Hall in Cooperstown.

The exhibit emerged out of conversations about the All-American league between Spencer and Ms. Pratt, whose suitcase is now part of the Hall’s permanent collection. Its opening was attended by Marshall, the filmmaker, who went on to interview former players and release “A League of Their Own” (1992), starring Tom Hanks, Geena Davis and Madonna.

Ms. Pratt was the last surviving member of the Peaches’ historic 1943 roster, according to the All-American’s Players Association. She never married and had no children, and died at a nursing home in Braintree, Mass., according to a paid obituary by her family, which did not give a precise cause.

Although Ms. Pratt’s memory was fading in recent years, she still knew all the words to the Rockford Peaches’ team song (“Oh, we’re all in bed by 10 o’clock, that is a dirty lie/ We are the Rockford ballclub, our motto ‘Do or die’ ”), which she sang for her 100th birthday party, according to Quincy’s Patriot Ledger newspaper.

In a moment of reflection, she added: “The good things I used to think about sports . . . memories, you live on with them, because baseball is baseball, whether it’s today, or tomorrow or 50 years ago.”