The original cover art for the first edition of “Harry Potter and the Philosopher’s Stone” sold for $1.92 million at auction on Wednesday, becoming the most expensive item related to the series, decades after its illustrator was paid a commission of just $650.

The watercolor painting, which depicts the young wizard Harry going to Hogwarts from Platform 9¾ at King’s Cross station, was part of the private library of an American book collector and surgeon, Dr. Rodney P. Swantko, whose other rare items were auctioned at Sotheby’s in New York this week.

The year before the novel came out in 1997, its publisher, Bloomsbury, hired a 23-year-old from England who had just graduated from art school to design the book jacket, the auction house said. The artist, Thomas Taylor, would go on to establish the world’s conception of Harry Potter, with his iconic round glasses and lightning bolt scar.

“It’s kind of staggering, really,” he said about the sale of his painting in an interview on Thursday. “It’s exciting to see it fought over.”

Mr. Taylor was working at a children’s bookstore when he submitted sample drawings of wizards and dragons for the publisher in London to review, he said in a 2022 podcast interview. When he was selected, he said, “I was over the moon.”

The cover was Mr. Taylor’s first professional assignment. And, at the time, “J.K. Rowling was as unknown as I was,” he wrote in his blog, referring to the novel’s British author.

Because neither the author nor the illustrator was well-known, Mr. Taylor was paid only 500 British pounds, he said in the interview Thursday. To celebrate, he wrote in his blog, he probably bought some new pens and a bottle of Belgian beer.

As he prepared to draw the cover, Mr. Taylor read an incomplete manuscript, threw around different ideas and made several drafts before arriving on a final design with an editor, he said in a 2010 interview with The Rowling Library, a website focused on the writer’s oeuvre.

He drew the artwork on a sheet of roughly 16- by 11-inch watercolor paper, and signed and dated it on the reverse side, the auction house said. Today, there are faint color variations and tape remnants on the edges, it said.

“The final image — a pencil sketch, painted with concentrated watercolor and then outlined with a black Karisma pencil — took two days,” Mr. Taylor said in the blog, adding that he delivered the finished work to the publisher by hand.

The drawing sold on Wednesday included more detail than was reproduced in the published cover, the auction house said. Platform 10 is visible in the artwork, for example, and it also showed more of Harry’s clothing and the steam from the smokestack of Hogwarts Express.

The book took off rapidly. In 1998, it landed in American bookstores as “Harry Potter and the Sorcerer’s Stone,” with a cover illustrated by another artist. In Britain, Bloomsbury moved on to more experienced illustrators, so Mr. Taylor’s drawing was his only contribution to the series, he wrote.

But he later negotiated a larger fee for the artwork, which was used for multiple overseas editions of the novel, he said. In 2001, when its film adaptation was being released, he decided to sell the illustration, thinking it would rise in value and fearing that he could not look after it properly and might spill coffee on it, he said. He sold it for about $100,000 at auction.

“I did make quite a lot of money in the end,” he said, adding that he was not upset that he missed out on the $1.92 million for which his drawing was sold more than 20 years later. “I’m just happy that I sold it when I did.”

Eventually, the novel became part of a world-famous series of seven books, translated into more than 70 languages, and the basis for some of the highest-grossing film franchises. This spring, Max, the streaming service from Warner Bros. Discovery, announced a television show based on the series.

“Harry Potter gave me a magical start to my career and I’m very grateful to him for that,” said Mr. Taylor, now an established author and illustrator of his own children’s mystery series called “Eerie-on-Sea.”

Just before the auction Wednesday, the artwork had been expected to sell for $400,000 to $600,000, the auction house said. It was eventually sold at Sotheby’s fine books and manuscripts sale in New York to an anonymous buyer.