Place lentils in a colander and rinse until the water runs clear. Combine lentils, 4 cups water, 1/2 teaspoon salt and turmeric in a large saucepan. Bring to a boil. Reduce heat to a simmer, partially cover and cook, stirring occasionally, for 15 minutes.
Meanwhile, heat oil in a large nonstick skillet over medium heat. Add cumin seeds and cook until fragrant and starting to brown, about 30 seconds. Add onion; cook, stirring, until soft and beginning to brown, 4 to 6 minutes. Add garlic, ginger, coriander, cayenne and the remaining 1/2 teaspoon salt and cook, stirring, for 1 minute more.
Stir the garlic mixture and mangoes into the lentils. Return to a simmer; cook, stirring occasionally, until the lentils are falling apart, 10 to 15 minutes more. Stir in cilantro.