The health and beauty retailer tested the range in 30 stores last Christmas and was pleased with how it performed.
Outside the US, the UK is the biggest market for Mary-Kate and Ashley products. Asda has been selling an exclusive range of Mary-Kate and Ashley clothing since 2002, while a more basics-driven range, exclusive to Littlewoods, was launched last year.
Dualstar Entertainment, the company owned by Mary-Kate and Ashley, expects to increase sales by 30 per cent to 50 per cent in the UK this year.
In March, Dualstar hired Diane Reichenberger as chief executive. She previously held executive positions at brands such as North Face, Gap and LA Gear.
Reichenberger refused to comment on UK sales of the Mary-Kate and Ashley brand, but it is understood that worldwide annual sales are more than US$1 billion (£545.3 million) a year.
She added that Mary-Kate and Ashley, who are in their freshman year at New York University, are looking at product areas and categories that could be launched by Dualstar. Reichenberger said that a fragrance range should hit UK shelves by the end of next year.