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She did have a heyday, though. From 1933-1938, she was a highly popular cover artist for two publications: Weird Tales, home to H.P. Lovecraft and Seabury Quinn, and Oriental Stories (later changed to The Magic Carpet). She also did fashion illustration work, although it is difficult to find.
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This art was dangerous, erotic, and highly popular, so much so that some of the magazine's contributing authors began writing certain scenes into their stories because they would make great Margaret Brundage cover art. There was some controversy eventually, because the women of Chicago and surrounding areas objected to such chauvinistic (and a little bit S&M) subject matter. But, as soon as the editor revealed that the artist behind these dramatic scenes was a woman, everybody was okay with it.
Then, the worst happened: Chicago tried to get morals. The mayor put certain "decency standards" into place, which meant that nude pulp fiction covers were no longer appropriate for newsstands and bookstores.
Weird Tales relocated to New York City (just try to clean up those morals!), and Brundage was unable to move with them. Since she worked solely with pastels on illustration board, which couldn't survive being shipped from Chicago to New York, she and the magazine had to part ways. They found local New York illustrators, and she was pretty much out of a job. She did sporadic freelance work, and a few sci-fi conventions and art shows (where several of her originals were stolen), but in the world of illustration, she was expendable.
*All information from Wikipedia