The Gossip Girl actress told OK that she was impressed with how well the pair got along while working.
"It's a very collaborative relationship. When I worked with them they were in the mode of putting things together," she said. "They agree on everything. They both have such great fashion sense. To come together on a line, I think is such an awesome thing. It really is something great. They did something great."
Momsen, who is the public face of the 'Material Girl' brand, noted that she was drawn to the fashion line because of Madonna and her daughter's shared desire to capture personal style.
"The cool thing about 'Material Girl' is it can fit into anyone’s personal style. It fits mine that is different than some girls and you can have a girly girl like my sister [who] would love the line as well as I do."
She continued: "The cool thing about it is it's very much for any girl. It's about the individual pieces and you put them together to make them your own. It covers a giant basis of styles, range; it's for any girl who wants to express themselves."
The Pretty Reckless singer further added that she was pleased to be representing a brand that is headed by the 'Vogue' singer.
"It’s an honour. I mean, the biggest honour," Momsen said.