In a small saucepan, stir together balsamic vinegar and packed brown sugar until sugar dissolves. Heat over medium heat until bubbly. Simmer, stirring constantly until vinegar mixture becomes thick and syrupy, 6-8 minutes. Set aside to cool.
Cut watermelon in half. Cut thick slices of watermelon; cut each slice into 4 triangles.
Arrange watermelon triangles on a serving platter. Drizzle with balsamic reduction, sprinkle with cheese and mint.