Wash the sago and soak it in ample water for about 30 minutes. Drain and keep aside.
Bring 4 cups of water to a boil and add the sago to it and cook till it turns translucent stirring every now & then to avoid scorching. Sago tends to stick to the pan very easily. Once done, drain the water and refresh with cold water. Keep aside.
Place the milk, coconut milk & sugar in a heavy-based saucepan and bring the mixture to a boil and stir until the sugar has dissolved. Turn down the heat, add the cooked sago and simmer for 3-4 minutes.
Check the consistency of the pudding. Do note that the pudding will thicken on its own due to the presence of starch in the sago. However, if you still desire a thicker pudding, add the cornstarch dissolved in the water and cook for half a minute. Optional step: Add the chopped mango pieces just before taking the pan off the heat.
Allow the pudding to cool slightly before serving if you want to serve it warm. However, it tastes better when chilled. Pour the pudding into serving glasses/bowls and cover with cling film to prevent a thick skin (cream) from forming. Once cool, refrigerate for at least 4-6 hours before serving garnished with chopped mango pieces and mint leaves