McMillan won her first Scotland cap against Spain when she started in the first Women's Rugby World Cup Qualifier at
Scotstoun Stadium in November 2016. She has continued to play for the team, moving from Back Row to Second Row, which she played for the first time in 2019. She was in Scotland Women U19 7s in 2013 and selected for the U20 squad, alongside her sister Siobhan, in 2014/15. She was in the Scotland Women 7s squad, which won the Rugby Europe Trophy in 2015 to gain promotion to the Grand Prix competition. A low point for McMillan came in January 2019 when she broke her scapula in a match away to Spain in Madrid. In the 2021 Women's Six Nations Championship match against England, the team lost 52–10 to the championship winners. This was her first outing as a lock for the Scottish team. They went on to take fifth place in the championship. She was "sin binned" in the match against Italy in the closing minutes for collapsing a maul that was creeping towards the line and that resulted in a penalty try. They lost the match 41-20 and went on to beat Wales. During the championship, McMillan commented on the Scottish team's improved performance from the previous year. She is a member of the
Scottish Rugby Academy and trains with the national team at Scotstoun Stadium. In the early stages of her international career she was described by The Scotsman as "an exciting back-row prospect" in a profile of the player. == Personal life ==