In a large bowl, combine the rice, cinnamon, and 4 cups of water and allow this to soak for at least 4 hours, but preferably overnight.
Once soaked, add the rice, cinnamon, and soaking water to a blender along with half of the chopped mango and blend until smooth.
Strain the blended mixture into a large pitcher to remove any remaining solids.
Add the sweetened condensed milk, evaporated milk, vanilla extract, milk, 2 additional cups of water, sugar, and about two-thirds of the remaining chopped mango to the pitcher.
Stir everything well to combine.
Serve the mango horchata in individual glasses filled with ice and top each serving with some of the reserved chopped mango.
Enjoy!