LUCY MERCER MISTRESS OF U.S. PRESIDENT FRANKLIN ROOSEVELT)

Lucy Mercer was hired by Franklin Roosevelt's wife Eleanor, to be her personal assistant, and there is no doubt that she had an affair with Franklin, a married man, while she was unmarried herself, and that Eleanor found out about it and was devastated.

But there is no evidence that Lucy was an actual mistress, if your definition of Mistress is a Kept Woman. She went on to marry someone else. And Franklin and Eleanor, however unhappy they might have been in their marriage, whether or not Eleanor had an affair or emotional entanglement with another man or another woman or not, they stayed married. But after the death of her husband, Lucy reentered his life... so perhaps she as his love of his life and he was hers - despite the marriages.

Click on the title above to get to a New York Times article that includes a photo and mentions one of the books about their relationship.