In a food processor or blender, puree mango slices. In a medium saucepan combine mango puree, brown sugar, and vinegar. Cook, stirring occasionally, until mixture comes to boiling. Reduce heat and simmer, uncovered, 15 minutes, stirring occasionally.
In a small bowl combine water and cornstarch; stir into mango mixture. Cook, stirring frequently, for 10 minutes or until mixture is the consistency of thin ketchup. Cool. Cover and store in refrigerator for up to 3 days (mixture will thicken more during chilling). Makes 2 cups.