Weir joined FA WSL side Arsenal Ladies in July 2013 after leaving high school. Whilst at Arsenal Weir won the 2014 FA Women's Cup.
Having represented Scotland at all youth levels, Weir received her first call-up for the Scotland women's senior squad for their match against Iceland in June 2013. She retained her place in the squad for the following game against Germany. During Euro 2017, Weir scored Scotland's opening goal against Spain and was voted player of the game.