Louise was the natural daughter of Louis de Marillac, her birth falling between his two marriages. However, he acknowledged her and paid for her education in a comfortable convent as she was growing up. After his death, the money stopped flowing and she was outplaced to more humble circumstances -- maybe her mom's cottage.
She had hoped to become a nun, but poor health and poor purse made a poor candidate, she she married Antoine de Gras, a secretary in the household of the French Queen. A series of tough breaks followed, including a dim-witted son, her husband losing his job, then losing his health, then his money in a financial scheme, and finally his life. She figured all this bad fortune resulted from her failure to be a nun, so she sought spiritual advice from Francis de Sales and Pierre Camus. They handed her off to Vincent de Paul, who pondered her situation for a while.
His eventual solution was asking her to survey the Ladies of Charity and recommend improvements in their efforts. She dove into her work, eventually transforming the benevolent but disorganized benevolence of aristocratic women into the Sisters of Charity, an urban service order without a cloister, convent, or chapel.