Dan Reilly, who made mascot history when he bravely donned an unventilated, oversize papier-mâché head, with simulated stitches, to become the New York Mets’ first Mr. Met, died on Dec. 30 in Manhattan. He was 83.

His son, Matt MacCartney, his only immediate survivor, said the cause was a heart attack while he was rehabilitating from pneumonia.

Mr. Reilly was working in the Mets’ ticket office when two team executives asked him to breathe corporeal life into Mr. Met, who had existed until then only as a cartoonish image used on promotional material.

On May 31, 1964, Mr. Reilly officially slipped on his Mets uniform and baseball-shaped head for the first time during a doubleheader at Shea Stadium against the San Francisco Giants.

“They had told me to play it straight, just walk out there and wave, but the kids started swarming down to meet me in the stands,” Mr. Reilly told The Amazing Shea Stadium Autograph Project, a blog. “I shook hands, posed for pictures, signed autographs. After that, I got cocky and started dancing. It was an instant hit.”

It was a better day for Mr. Reilly than it was for the Mets, who were swept by the Giants (the second game required 23 innings; the Giants won, 8-6) — a typical day for the perennially losing team then in its third season as an expansion franchise.

Later that year, Mr. Met appeared in a Mets event at the World’s Fair in Flushing, near Shea Stadium. Mr. Reilly sat at the edge of the Unisphere, the giant globe that was the centerpiece of the fair, getting splattered by its water display and signing “Mr. Met” for a procession of children.

“Who would know Dan Reilly?” he observed to The Daily News that day.

The Mets sidelined Mr. Met, but not Mr. Reilly, in 1967, and waited a dozen years to bring in another mascot species: Mettle, a mule (a real one), who strode up and down the foul lines before games during the miserable 1979 season, when the Mets won 63 games and lost 99. Mr. Met was revived in 1994 (with a man named A.J. Mass soloing under the colossal noggin until 1997) and has appeared ever since, even showing up in ESPN ads and commercials for various products.

“From Dan Reilly until today, Mr. Met has been a symbol of the Mets who is really valued by the team and its fans,” Dave Raymond, the first person to portray the Phillie Phanatic, the mischievous mascot of the Philadelphia Phillies, said in an interview. “He reflects the passion, angst, affection and frustrations of Mets fans.”

Although Mr. Met was the first costumed mascot to achieve prominence in Major League Baseball, Mr. Raymond said, he was not the first: He was preceded in the 1950s by the little-remembered Mr. Oriole, a costumed bird who worked for the Baltimore Orioles.

Mr. Met (not Mr. Reilly) was inducted into the Mascot Hall of Fame in Whiting, Ind., in 2007.

Daniel Joseph Reilly was born on Jan. 28, 1938, in Richmond Hill, Queens, to Anna and Robert Reilly. After graduating from high school, he attended broadcasting school, served in the Marines in Guam and became a ticket agent for Eastern Airlines.

After about a decade with the Mets, he worked in promotions for the New York (now Brooklyn) Nets and Colonie Hill Golf Club (now the Wind Watch Golf & Country Club), in Hauppauge, on Long Island; as a bartender; and as a public address announcer for New York Waterways. On one of his routes he offered his Mr. Met memories to fans traveling from ports in New Jersey and Manhattan to the World’s Fair Marina, from which they walked to Shea Stadium.

He married Gloria Westerweller on the afternoon of Oct. 11, 1969, the day of Game 1 of the World Series between the Mets and the Baltimore Orioles. It was a road game, which prevented his closest friends on the Mets, the outfielder Ron Swoboda and the pitcher Tug McGraw, from attending; the arrangements had been made long before the Mets were expected to be postseason contenders.

That marriage ended in divorce. His second marriage ended with his wife’s death.

In 2007, Mr. Reilly wrote a book, “The Original Mr. Met Remembers: When the Miracle Began,” in which he reminisced about his time under the big giant head and reflected on his legacy.

“Today, baseball fans know about the Phillie Phanatic and the San Diego Chicken,” he wrote. “The Kansas City Royals have their Sluggerrr and the Cardinals have their Fredbird. All of them are descended from the original Mr. Met.”