Bring a large saucepan of water to a boil. Add the carrots and cook for 2 minutes; drain and run under cold water to cool. Transfer to a bowl.
Meanwhile, in a small saucepan, combine the vinegar, sugar, coriander and mustard seeds, ½ teaspoon salt, and 1 ½ cups water and bring to a boil. Pour over the carrots and let cool to room temperature. Add the mint and refrigerate for at least 2 hours.