In 1890, just five years after William Le Baron Jenney invented the world’s first skyscraper, the average workweek for full-time US manufacturing employees was 100 hours. Divided over seven days, such a worker would have been putting in 14-hour shifts every day in the hopes of giving their family a decent life. Keep in mind, this was also a time before child labor laws were set in place, meaning thousands of children were also working long days, doing dangerous work.
While many states and cities made an effort to standardize the 40-hour workweek or even the eight-hour workday prior to 1938, they received pushback from employers. In fact, after the Illinois Legislature mandated an eight-hour workday in 1867, many employers refused to adapt, prompting workers to strike in such numbers. The day (May 1) became known as May Day.
The movement had been a long time coming. Twenty-one years prior, after President Ulysses S. Grant issued an 1869 proclamation giving government workers steady wages and an eight-hour workday, employees outside the public sector renewed their efforts, fighting for the eight-hour workday. A few private companies, including Ford Motor Companies, began to set the tone, giving workers five-day, 40-hour workweeks.
Finally, in 1938, the Fair Labor Standards Act (FLSA) was passed into law, limiting workweeks to 44 hours — later amended to 40 hours in 1940.
President Roosevelt sent the bill to Congress, saying, “Our nation so richly endowed with natural resources and with a capable and industrious population should be able to devise ways and means of insuring to all our able-bodied working men and women a fair day's pay for a fair day's work. A self-supporting and self-respecting democracy can plead no justification for the existence of child labor, no economic reason for chiseling workers' wages or stretching workers' hours.”
While the law that was passed ended up being quite a bit weaker than originally intended, it did limit child labor within the United States and introduce time-and-a-half overtime pay for certain jobs.
Read more about the history of the FLSA and the story behind its passage, from dol.gov.