Mannion was called-up to represent England at the 2012 and 2013 UEFA Women's Under-19 Championship. She played every minute as England reached the final of the latter before losing to France. Despite the loss, the result qualified the team for the 2014 FIFA U-20 Women's World Cup in Canada. Mannion again played every minute as draws against South Korea and Mexico before defeat against Nigeria saw the team finish third in the group and eliminated.