The twin actresses - who have reportedly brought in over $1 billion via their Dualstar company's retail ventures - admitted that they struggled to find acceptance in the clothing industry for their three brands The Row, Elizabeth & James and Olsenboye.
"People would drill us about fabric, where we'd make it," Mary-Kate told Vogue of their initial collection.
"The first season, customers bought it, so the stores came back. And drilled us again. And we were really designing it. It would be jeans, a bit bohemian, or with a little blazer. It was really fashion-forward."
Ashley added: "It's a fair question but Dualstar started when we were six.
"And we had a collection with Walmart at 12, which was the upper tier of the tween market. It was before celebrity designers."
The sisters were inducted into the Council Of Fashion Designers Of America in 2009.
Ashley previously considered leaving fashion to return to her acting career.