Bring 2 cups of water to a rolling boil in a medium saucepan. Add the sago pearls and reduce the heat to a gentle simmer, stirring constantly for 1 minute to prevent sticking. Cook for 15 minutes, stirring occasionally, until the pearls become mostly translucent with just a small white dot in the center.
Remove the saucepan from the heat, cover with a lid, and let it stand for 10 minutes to allow the sago to become fully transparent and soft. Drain the sago through a fine-mesh sieve and rinse under cold running water until completely cool to the touch.
While the sago cools, peel and cube the mangoes. Reserve 1 cup of the mango cubes for garnish and place the remaining mango in a blender.
Add the granulated sugar, coconut milk, and evaporated milk to the blender with the mango. Blend on high speed for 60 seconds until the mixture is completely smooth and creamy.
Pour the mango milk mixture into a large bowl or pitcher. Stir in the cooled, drained sago pearls until they are evenly distributed throughout the mixture.
Cover the bowl and refrigerate for at least 1 hour to chill thoroughly. Serve cold in individual bowls, topped with the reserved fresh mango cubes and over ice if desired.