The Amalgamated Clothing Workers of America, the most significant union representing workers in the men's clothing industry, was founded in New York City in 1914 as a breakaway movement from the United Garment Workers. Radical and immigrant workers in the tailors’ and cutters’ locals were the core of the seceding group, which advocated industrial unionism and economic strikes in opposition to the UGW’s craft organization, which they saw as conservative and timid. Their diverging views had come to the fore during the historic 1910 dispute at the Chicago firm Hart, Schaffner, and Marx. The opposition called the strike against the UGW leadership’s advice, and reached a path-breaking agreement with management that established an arbitration system to settle disputes.

Members flocked to the new union. Around 50,000 strong at its founding, by 1920 the ACWA counted about 170,000 members. Initially composed mostly of immigrants of Jewish European descent with Socialist leanings, the ACWA quickly welcomed members of a great number of nationalities and diverse backgrounds. Like in other garment unions, most workers and many members were women, but the leadership was predominantly male, a situation that did not change for many decades. Early on the union adopted a centralized administrative structure combined with industrial unionism, with the joint boards’ by-laws having precedence over those of locals.

Espousing a philosophy perhaps brought over by its early immigrant socialist members, the Amalgamated went beyond bread and butter issues and adopted a distinctive form of social unionism that was largely absent in the American labor movement. Starting in the 1920s, it provided educational opportunities and recreational facilities for its members, as well as services such as an insurance plan, banks offering personal loans at low interest rates, low-cost housing cooperatives, medical clinics, and even union-owned restaurants.

Sidney Hillman was the first president of the new union and the most important officer in its history. He applied his experience as bargaining representative in Chicago to the whole industry. Under his leadership the union made significant strides in securing better wages and working conditions for its members, and at the same time it consolidated gains and provided stability to the industry through the widespread adoption of the arbitration system tested at Hart, Schaffner, and Marx. Hillman paid close attention to industry issues, such as production, pricing, and marketing. In order to help management meet the competition of non-union firms, the union conducted studies of efficiency, work methods, and factory costs. Letters to the official publication of the union, Advance, document the controversy that ensued within the union over what was perceived to be collaboration with management.

Hillman also understood the importance of labor’s involvement in national affairs and political action. In the 1920s the ACWA sent delegates to the Conference for Progressive Political Action and to the Farmer-labor party conventions. Although many members and officers were Socialists, the union stopped short of officially endorsing the party. Communist attempts at gaining influence within the union were firmly curbed. Hillman’s participation in national affairs and politics became prominent during the New Deal, when he became a close advisor to Franklin D. Roosevelt on labor and economic issues. He also served on the board of the National Recovery Administration. Later, during World War II, he helped establish the Labor’s Non Partisan League. He was also named associate director of the Office of Production Management, which assisted in mobilizing the nation's resources for the war effort. Hillman’s prestige perhaps reflected the healthy condition of his union, which by the end of the conflict was strong and stable.

During the post World War II period the union faced a number of significant challenges. Membership continued to grow (peaking at 395,000 in 1968), but the union’s political influence and visibility in national affairs declined. In their never ending pursuit of lower production costs, many firms relocated to the South, forcing the union to engage in large organizing efforts. Simultaneously, signs began to appear of changes that would lead to the almost complete demise of the domestic apparel industry and, ultimately, to the erosion of union membership. Foreign imports of cheap clothing goods steadily grew in the 1950s and 1960s, and mushroomed in the following two decades, plunging employment in the apparel sector into a steady decline. Union efforts to stem the tide included Buy American campaigns and extensive lobbying in Congress, but they were to no avail. In 1976, the ACWA merged with the Textile Workers of America to become the Amalgamated Clothing and Textile Workers Union. Despite successful and much publicized nationwide actions such as the Farah boycott and the J.P. Stevens corporate campaign, the woes threatening the union’s existence continued unabated. The fate of the domestic industry was sealed in the late 1970s and the 1980s by the flight of firms chasing tax breaks and cheap labor abroad. By 1995, when ACTWU voted to merge with the International Ladies' Garment Workers' Union, their combined membership was 350,000. The new Union of Needletrades, Industrial and Textile Employees (UNITE!) seemed poised to infuse new life in a troubled union.