Triangle begins in New York City in 1909, two years before the Triangle Shirtwaist Factory fire, when a young seamstress and strike leader named Clara Lemlich was followed and beaten by hired thugs due to her political activities. A member of the International Ladies' Garment Workers' Union (ILGWU), Lemlich transformed her beating into a call for change, becoming a martyr of the movement and galvanizing other workers to strike. Von Drehle argues that "what begins with Clara Lemlich's beating leads to the ravenous flames inside the Triangle Waist Company" that transformed the political machinery of New York.

New York City experienced an enormous influx of immigrants in the early twentieth century, and many women entered the workforce. Young immigrants organized themselves to demand "a more fair and humane society," and factory owners often hired strikebreakers to forcibly end the protests. Von Drehle labels the summer of 1909 the "season of strikes," wherein workers successfully advocated for better pay and working conditions. News of the success of the Rosen Bros. Strike—in which workers walked out of one of the largest shirtwaist factories in the city and refused to negotiate or return to work without a twenty percent pay raise—quickly spread throughout the garment district and prompted other workers to walk out, too.

The early years of the twentieth century also saw the growth of progressivism, a movement that supported votes for women, protection for both workers and consumers, and the organization of workers into trade unions. Two weeks after Clara Lemlich’s beating, a festival honoring Henry Hudson drew a crowd two million strong. An armada of ships and gunboats representing nations from all over the world sailed down the Hudson River, and a dazzling electric light show took place that evening. For New Yorkers, the spectacle confirmed that "progress wasn't a theory—it was everywhere they turned." Progressivism was gaining momentum, with President Theodore Roosevelt the "embodiment of pure possibility" and New York's governor pushing progressive reforms in Albany.

Tammany Hall, which was the dominant political machine in New York for half a century, had historically opposed progressivism. Controlled by the Democratic Party and known for its involvement in corruption and underhanded politics, Tammany relied on a network of errand boys and block captains. In return for goodwill and assistance for those in dire need, they asked for votes: "Tammany's little kindnesses solved no social problems, but they did help people through difficult times." As a result, they garnered an enormous amount of loyalty, particularly among poor immigrants, who were entering New York at unprecedented rates.

Tammany Hall's leader was Charles Murphy, who realized that the old order of corruption and favors was not enticing "New Immigrants" to support Tammany. These were predominantly Eastern European Jews and Italians, who were arriving in New York in large numbers. Murphy focused on gaining the support of the Eastern Europeans, as they were much more politically active than Italians, who rarely voted, and therefore of more value to Tammany Hall. Progress was inevitable, and Tammany had to take part in it if they wanted to survive. The system of personal gestures that had served them so effectively was failing to sway the New Immigrants, and Murphy was only just able to secure a victory for Tammany’s incumbent mayor, George B. McClellan Jr., in the 1905 mayoral contest. Despite his ostentatious capitalist sensibilities, Hearst garnered working-class and immigrant support and vowed to fight corruption head-on. In 1909, as Hearst ran for mayor once again, Murphy was likely being informed of Clara Lemlich's activities and the unrest in the garment district.

That same year saw the opening of Israel Zangwill's play about the immigrant experience, The Melting Pot. It was an affordable theater experience, with "the cheapest tickets within reach of all but the lowest-paid shop girls," and became an overnight sensation. Garment workers saw themselves represented onstage, and the title quickly became a ubiquitous metaphor for the mixing of immigrant cultures in the United States.