Peel mangos and remove the seed. Cut up into large chunks and add to a heavy stock pot.
Add sugar and stir with a wooden spoon.
On medium-high heat begin to cook down the mixture. Once it is bubbling turn down the temperature to medium-low. Allow the mixture to reduce about 40-60 minutes. Stir regularly to keep the mixture from sticking or scorching. If the mangos are very watery it may take longer to reduce to a jam-like consistency. Another 20-40 minutes.
Ladle the mango jam into mason jars and allow to cool before covering. Once cooled to room temperature you can place the cover on the jars and store in the refrigerator. Will keep for several weeks in the refrigerator.