The Supreme Court struck it down in 1936.
Social-justice crusaders erupted in protest, circulating proposals to strengthen the rights of states to pass legislation of their choosing. When a State Supreme Court upholds a law, they suggested, a super-majority of Supreme Court justices, or even unanimity, should be required to strike it down.
President Roosevelt had already been tangling with the court's conservatives every time that they struck down one of his New Deal laws as an unconstitutional impingement on economic liberty. You'll recall President Roosevelt's brazen court packing threat and its apparent success influencing the justices. As it turned out, striking down the New York minimum wage law in 1936 was the anti-New Deal faction's last stand. A year later, an only slightly changed court reversed itself, upholding a Washington State minimum-wage law. Associate Justice Owen Roberts' uncharacteristic vote formed the 5-to-4 majority dubbed "the switch in time that saved nine."
That case was decided in 1937.
The same year, FDR declared, "All but the hopeless reactionary will agree that to conserve our primary resource of manpower, government must have some control over maximum hours, minimum wages, the evil of child labor, and the exploitation of unorganized labor."
But calling for "minimum wages" still implicitly meant "for women," at least among the political mainstream.
A Collier's editorial published May 8, 1937, is instructive. Titled "A Woman's Work," it lauds social reformer Florence Kelley for her early minimum-wage advocacy before sketching a brief history of the movement. "The late Samuel Gompers and labor union leaders generally were opposed to the idea so far as it might affect men. Mr. Gompers thought that unions could get more by bargaining than the states would grant through minimum wage laws," the editorial stated. "The American Federation of Labor agreed with him. Unions still oppose minimum wage laws for men. Long ago, however, they conceded the desirability of such legislation for women."
It goes on to explain that support, which was grounded in economic self-interest as much as any reformer's impulse:
The justification for minimum wage laws is found in the appallingly low rates of pay obtained by women in some industries. This year, for instance, when times are generally good, according to New York's industrial commissioner, Elmer F. Andrews, "thousands of women are being paid less than 20 cents an hour and a great many are receiving less than 10 cents an hour" in New York, which boasts the highest age levels in the country!
Commissioner Andrews added these significant words: "In a substantial number of instances women are working long hours at wages as low as two and three cents an hour. Some of this low-paid work is highly skilled. These low wages exert a downward suction on the wages and working conditions of every wage earner in the state.
The editorial goes on to say something that surprises me every time I reread it: "Except as the NRA, which was declared unconstitutional by the Supreme Court, set up a minimum wage for all employed, no effort has so far been made to enact similar laws for men. Labor unions have continued their opposition." But it went on to note that organized labor was changing very rapidly at the time, and that "it is entirely possible that demands may be made for the establishment of legal pay rates for men." What wasn't clear is how the Supreme Court would react. In its 1937 decision deeming a minimum wage constitutional, the justices again emphasizing the differences in the sexes. The state has a special interest in protecting women, it declared:
That phase of the subject received elaborate consideration in Muller v. Oregon, where the constitutional authority of the state to limit the working hours of women was sustained. We emphasized the consideration that 'woman's physical structure and the performance of maternal functions place her at a disadvantage in the struggle for subsistence' and that her physical well being 'becomes an object of public interest and care in order to preserve the strength and vigor of the race.' We emphasized the need of protecting women against oppression despite her possession of contractual rights. We said that 'though limitations upon personal and contractual rights may be removed by legislation, there is that in her disposition and habits of life which will operate against a full assertion of those rights. She will still be where some legislation to protect her seems necessary to secure a real equality of right.' Hence she was 'properly placed in a class by herself, and legislation designed for her protection may be sustained, even when like legislation is not necessary for men, and could not be sustained.'
Limiting women's workdays to 10 hours in Muller, the court had declared it constitutional as a consequence of their inferiority. Striking down the minimum wage 15 years later, it affirmed women's equality. In reversing that decision, so that the minimum wage was extended to women, it also reversed itself once more on the subject of their standing, relative to men, in the world of work. Then, in 1938, Roosevelt signed into law a bill that established a minimum wage of 25 cents per hour for men and women. It was never invalidated by the courts. And quite suddenly, the longstanding connection between the minimum wage and women's inequality was severed.