Tom Seaver, a Hall of Fame pitcher and the hero of New York’s “Miracle Mets,” who led his once-hapless team from the National League basement to an improbable World Series championship in 1969, died Aug. 31 at age 75.

The National Baseball Hall of Fame announced his death, noting that the causes were Lewy body dementia and covid-19, the disease caused by the novel coronavirus. The statement did not say where he died. Mr. Seaver and his family announced in 2019 that he was withdrawing from public life because of advancing dementia.

From the time he came to New York in 1967 as a 22-year-old rookie, Mr. Seaver began to transform a team that had been known as an inept group of lovable losers since the franchise began five years earlier.

Enduring another defeat, the team’s first manager, Casey Stengel, memorably quipped, “Can’t anybody here play this game?”

The clean-cut Mr. Seaver, who had been to college and served in the Marine Corps, brought an orderly sense of purpose to his pitching and to the Mets organization.

In high school, the 5-foot-9, 160-pound Mr. Seaver learned to pitch with finesse, precision and determination. Only later, after he grew four inches and gained 40 pounds, did he have the ability to throw the blazing, pinpoint-accurate fastball that made him one of the most dominant pitchers of his era.

He is generally ranked among the 10 best pitchers in history by baseball historians.

“There is a good argument that Tom Seaver is the greatest pitcher of all time,” noted statistical guru Bill James.

Pitchers are among the most single-minded of athletes, and few were as devoted to their craft as Mr. Seaver. He always wore a long-sleeve shirt at the beach to keep his arm from being sunburned. When petting a dog, he used his left hand, not his right, or throwing, hand. He brought an intellectual approach to pitching that often led sportswriters to describe him as an artist on the mound.

“Pitching is what makes me happy,” he told Sports Illustrated in 1972. “I’ve devoted my life to it. I live my life around the four days between starts. It determines what I eat, when I go to bed, what I do when I’m awake. It determines how I spend my life when I’m not pitching.”

During his rookie season, the Mets finished in last place, a dismal 61-101, but he was a lone beacon of excellence, with a record of 16-13. He pitched in the All-Star Game that year, saving a 15-inning victory for the National League. He was nicknamed “Tom Terrific” and “The Franchise.”

The next year, under new manager Gil Hodges, a onetime star with the Brooklyn Dodgers, the Mets improved. Their breakthrough came in 1969, when they surprised the rest of the league, overtaking one team after another as they rose from cellar to first place.

The team’s top hitters were Cleon Jones and Tommie Agee, but the Mets were built around a young pitching staff that included Gary Gentry, left-hander Jerry Koosman, future Hall of Famer Nolan Ryan and reliever Tug McGraw. But no player on the Amazin’ Mets, as they became known, was more amazing than Mr. Seaver.

“Tom Seaver was the driving force behind the players, always pushing the team to be better than they were, never letting them settle,” Hall of Fame player and Mets broadcaster Ralph Kiner wrote in his 2005 memoir, “Baseball Forever.”

The 1969 season was the first in which the league was separated into two divisions. Early in the year, the Mets struggled and in late May were in fifth place in the six-team East division.

By July 9, the Mets had climbed to second place, and Mr. Seaver faced the division-leading Chicago Cubs at Shea Stadium. Against a lineup that included three future Hall of Famers — Ernie Banks, Ron Santo and Billy Williams — Mr. Seaver was unhittable. He retired every Cub he faced through eight innings.

Finally, with one out in the ninth, a little-used utility player named Jim Qualls poked a single to left-center field for the Cubs’ only hit and only base runner of the game. Mr. Seaver then retired the final two hitters for what he called his “imperfect game,” one of five one-hitters he pitched in his career.

On the mound, Mr. Seaver may have lacked the classic elegance of Jim Palmer of the Baltimore Orioles, the high-kicking artistry of the San Francisco Giants’ Juan Marichal or the intense glare of the St. Louis Cardinals’ Bob Gibson. Instead, he relied on his thick legs and trunk to generate the power needed for his “drop-and-drive” delivery. His motion was so compact that he almost seemed to throw his pitches — a fastball, slider and curve — from a crouch. He dipped his trailing (right) leg so low that he wore a pad on his knee for protection because he often scraped it on the dirt on the pitcher’s mound.

After Aug. 5, Mr. Seaver did not lose a game for the rest of the 1969 season. On Sept. 9, during a stretch when his team won 13 of 14 games, he mowed down the Cubs again, 7-1.

The next day, the Mets won a doubleheader over the Montreal Expos to take over first place in the National League East. They never trailed again, winning 38 of their final 49 games to finish with a record of 100-62.

Mr. Seaver won his last 10 decisions, finishing with a 25-7 record and 2.21 ERA.

“God is a Met,” Mr. Seaver said at the time. “I heard that somewhere. Now I believe it.”

The Mets swept the Atlanta Braves in the National League playoffs, and then faced the Baltimore Orioles in the World Series. Led by pitching aces Palmer, Dave McNally and Mike Cuellar, and with a lineup that included Frank Robinson, Brooks Robinson and Boog Powell, the Orioles had won a team-record 109 games and were the overwhelming favorite in the Series.

Mr. Seaver started the first game in Baltimore but lost to Cuellar and the Orioles, 4-1. The Mets won the next two games, and Mr. Seaver took the mound for Game 4 before more than 57,000 fans in Shea Stadium.

In the top of the ninth inning, the Mets’ ungainly right fielder, Ron Swoboda, made a diving catch of a line drive off the bat of Brooks Robinson to stop a Baltimore rally. The score was tied 1-1 after nine innings.

Mr. Seaver came out to pitch the 10th inning, allowing two base runners before striking out the Orioles’ Paul Blair to end the threat. In the bottom of the inning, the Mets pushed across a run to win the game, 2-1. The next day, behind Koosman, the Mets won again, 5-3, to claim their first World Series victory.

Mr. Seaver joined the team in a ticker-tape parade down Broadway. He was named Sports Illustrated magazine’s sportsman of the year and won the Cy Young Award as the National League’s top pitcher.

He won his second Cy Young in 1973, as he led the Mets to another World Series, which they lost to the Oakland Athletics in seven games. He took home a third Cy Young in 1975, becoming only the second pitcher, after Sandy Koufax, to win the award three times. (That total was later surpassed by Roger Clemens, who won seven Cy Youngs, Randy Johnson, with five, and Steve Carlton and Greg Maddux, who won four each.)

In 1974, Mr. Seaver was baseball’s highest-paid pitcher, earning $170,000 a year. As salaries began to soar with the advent of free agency, he remained a relative bargain but was also an outspoken member of the players’ union. During a contract dispute in 1977, the Mets traded him to the Cincinnati Reds, incurring the wrath of New York fans and sportswriters.

The next year, Mr. Seaver pitched a no-hitter for the Reds — the only one of his career — and in 1981 he finished second in the Cy Young voting, with a 14-2 record during a strike-shortened season. He returned to the Mets in 1983, and then spent parts of three seasons with the Chicago White Sox, for whom he won his milestone 300th game in 1985. He ended his career with the pennant-winning Boston Red Sox in 1986. (A knee injury kept him from playing in the World Series against the Mets.)

Mr. Seaver retired with 311 victories and a dazzling 2.86 ERA. He led the league in strikeouts five times, and victories and ERA three times each, including a career-best ERA of 1.76 in 1971. His 61 shutouts are tied with Ryan, his onetime teammate, for seventh most in baseball history.

The Mets had previously retired the numbers of managers Stengel and Hodges, but Mr. Seaver was the first player whose uniform (No. 41) was retired by the team. In 1992, he was elected to the National Baseball Hall of Fame with 98.8 percent of the vote, a record at the time.

“People ask me after a shutout, ‘Was that fun?’ ” he told Sports Illustrated in 1981. “And the answer is no. No, it was not fun. Because fun is such a minuscule word for the satisfaction of what I’m doing.”

George Thomas Seaver was born Nov. 17, 1944, in Fresno, Calif. His father, an executive with a raisin-packing company, played football, basketball and golf at Stanford University. His mother, brother and two sisters were also athletic.

After high school, Mr. Seaver joined the Marine Corps Reserve, and then attended a community college in Fresno before receiving an athletic scholarship to the University of Southern California, where he was a pitching standout for one year.

He was declared ineligible for his senior season because he signed a contract to join the Atlanta Braves after college. Baseball Commissioner William Eckert nullified the contract and allowed other teams to bid for Mr. Seaver; the Mets won a lottery drawing in 1966.

During his one season in the minor leagues, his manager, Solly Hemus, said, “Seaver has a 35-year-old head on top of a 21-year-old body.”

Mr. Seaver was known for his wide-ranging interests beyond baseball, including books, investments, wine, art and gardening. He was among the first baseball players to voice public opposition to U.S. involvement in the Vietnam War.

Early in his career, Mr. Seaver traveled across the country in a mobile home with his wife, the former Nancy McIntyre, whom he credited with much of his success in baseball.

In 1974, Mr. Seaver completed his bachelor’s degree (in communications) at Southern Cal. After his playing career, he was a longtime broadcaster for the Mets, New York Yankees and NBC Sports. He lived in Connecticut for many years before settling in Calistoga, Calif., where he operated a vineyard.

Survivors include his wife of 54 years; two daughters; and four grandsons.

At the peak of his career, Mr. Seaver estimated that he could throw more than 95 percent of his pitches within an inch of his intended target.

“Pitching is a physical art form, I think,” he said in 1981. “I haven’t yet come close to finding anything as rewarding as pitching. Baseball is just so beautiful when it’s played right. God, but it can be a beautiful thing out there.”

An earlier version of this story incorrectly stated that 1968 was the first year the New York Mets did not finish in last place. They were ninth in the 10-team National League in 1966.