In Gilded Age New York City during the 1860s and 1870s, nobody wielded more political power than William Magear Tweed. Known by both his fans and fiercest critics as “Boss Tweed,” the former fireman rose through the ranks of New York’s Democratic party to pull the levers of the mighty political machine known as Tammany Hall.

Boss Tweed and his corrupt “Tweed Ring” of city officials siphoned millions of dollars from bloated public works projects like a lavish new courthouse that cost nearly $15 million to build, including $9 million in kickbacks going to Tweed and his cronies.

Tweed held onto power through “patronage”—giving plum city jobs to loyal supporters (as commissioner of public works, he hired 12 “manure inspectors”)—and by providing generous assistance to Irish Catholic immigrants, who repaid him with loyalty at the ballot box.

Boss Tweed operated with impunity—until he got under the skin of a 30-year-old political cartoonist named Thomas Nast. Nast launched a relentless anti-corruption campaign against Tweed in the pages of Harper’s Weekly. In his ferocious and funny caricatures, he painted Boss Tweed as a larger-than-life crook and Tammany Hall as a den of tigers.

Thanks in large part to Nast’s brutal cartoons and dogged reporting from an upstart newspaper called the New-York Times, Boss Tweed was finally brought to justice.

Thomas Nast was an immigrant himself. Born in Germany, nine-year-old Nast and his family arrived in New York City in 1846. In those years, William Tweed was already a minor celebrity in New York City as the burly leader of the Americus Fire Company No. 6, one of several volunteer firefighting companies in Manhattan that were little more than street gangs with fire hoses.

During the Civil War, young Nast sided with the “Radical Republicans” and put his artistic talents to work for the Union and abolitionist cause. When the odds were stacked heavily against Abraham Lincoln in the 1864 election, Nast published a two-page engraving called “Compromise with the South” that may have saved the beleaguered Republican president. In the 1868 election, Ulysses S. Grant credited his win to “the sword of Sheridan and the pencil of Nast.”

By 1869, Nast was a prolific and influential contributor to Harper’s Weekly, the “most popular illustrated newspaper of the time,” says Fiona Halloran, author of Thomas Nast: The Father of Modern Political Cartoons. That’s when Nast turned his attention to Boss Tweed and his Democratic Tammany Hall political machine.

“His whole life, Nast didn’t like hypocrisy and he had a very binary, black-and-white view of what was right and what was wrong,” says Halloran. “If someone was corrupt, that meant they were also a ‘really bad person,’ and Nast was gleeful about going after anyone who fell into that category. With Boss Tweed, Nast saw an opportunity to release a lot of venom in pursuit of something that would make him famous.”

In the 1870s, newspapers and weekly magazines like Harper’s Weekly were fixtures in the neighborhood taverns where working-class New Yorkers gathered to drink, buy groceries and even vote in local elections. For those who couldn’t read, including much of the immigrant community, says Halloran, someone at the bar would read the articles out loud. Political cartoons, including Nast’s brutal takedowns of Tweed, were pasted on the walls for everyone to see.

Nast produced more than 140 political cartoons targeting Boss Tweed, says Ryan Hyman, curator at the Macculloch Hall Historical Museum, which exhibits one of Nast’s most famous cartoons, “Tammany Tiger Loose—What are you going to do about it?” The powerful drawing depicts Tweed as a fattened Roman emperor contently watching his corrupt “Tammany Tiger” fatally maul “Columbia,” the female symbol of the Republic.

Nast drew inspiration for his cartoons from articles and editorials about Tweed’s brazen corruption published in the New-York Times, a new Republican newspaper. The more that the Times revealed, the angrier and bolder Nast’s drawings become. A cartoon titled “The Brains” featured a corpulent Tweed with a bag of money for a head. Another depicted all of New York under the giant thumb of Tweed.

The destructive potential of Nast’s cartoons wasn’t lost on Tweed.

“Let’s stop those damned pictures,” Tweed reportedly said. “I don’t care so much what the papers write about me—my constituents can’t read, but damn it, they can see pictures.”

In 1873, Nast was living in Harlem with his wife and small child when he says that a stranger knocked at their door with a suspicious question: “You’ve been working so hard on your cartoons, aren’t you tired? Wouldn’t like to go on a vacation?”

Nast recognized the man as one of Tweed’s lawyers and decided to play along, says Halloran. “What kind of vacation?” Nast asked. “Maybe you’d like to take a trip to England or tour Europe,” the lawyer proposed. “Well, I’d need a lot of money for that,” said Nast. “How much money? Would $100,000 be enough?” asked the lawyer.

According to Nast, he negotiated his payoff up to $500,000 before Tweed’s lawyer realized that Nast was messing with him and left with a threatening, “You’ll be sorry.”

Nast probably took this threat seriously, because he immediately moved his family from Harlem to Morristown, New Jersey, and bought a house across the street from historic Macculloch Hall. Hyman says that the museum’s collection includes 5,000 of Nast’s engravings and sketches, and some personal items as well.

“We have a walking stick in the collection donated by Thomas Nast’s son, Cyril,” says Hyman. “There’s a letter written along with it: ‘Dad carried this stick around during the Tweed campaign. It’s loaded with lead.’”

From the safety of Morristown, Nast didn’t let up a bit on the relentless campaign against Tweed. He churned out five or six cartoons a week for Harper’s.

“Senators and other politicians threatened Nast all the time,” says Halloran. “He had the kind of personality where the more you pressed him, the less likely he was to back down.”

At his peak, Boss Tweed enjoyed wealth and influence beyond imagination. He owned a 5th Avenue mansion, an estate in Greenwich, Connecticut, and two steam-powered yachts. In addition to his position as the Commissioner of Public Works, Tweed was the director of a bank, a railroad company and a publishing house.

Then the New-York Times finally caught up with him. The newspaper got its hands on a “smoking gun,” a secret Tammany Hall ledger detailing how Tweed and his “Ring” stole hand-over-fist from the city. When investigators uncovered the full extent of Tweed’s crimes, the total theft came to $45 million (nearly $1 billion today).

Ultimately, it was reporters and editors at the Times that took Tweed down, but Halloran says that Nast’s barrage of negative political cartoons had “an outsized effect on the campaign against Tweed. From the point of view of the ordinary Joe, it was Nast who toppled Tweed.”

Tweed was convicted of corruption in 1873 and died in prison four years later (after a failed escape attempt to Spain).

Nast, already well-known in Republican circles, became a national celebrity after the Tweed campaign. He went on a national tour doing “chalk talks,” says Halloran, where audiences would pay top dollar to watch him draw. Nast also could be racist in his work—he created demeaning caricatures of Irish-Americans and African Americans, presenting both groups as monkeys.

Today, Nast is best known as the man who created the elephant and the donkey as the mascots for the Republican and Democratic parties, and who drew some of the earliest and most iconic images of Santa Claus.