Millar-Mills made her debut for England in 2011 for the U20 team, before graduating to the senior team that year. In 2013 she played for the country in the 2013 Women's Six Nations Championships. In the game versus Scotland, she and her sister, Bridget Millar-Mills, were the first sisters ever to face each other in international rugby. Though both grew up in Manchester, Bridget chose to play for Scotland as their mother is from Hamilton.