Peel, seed and dice mangoes into 1/2 inch cubes. (Place a few small plates into the freezer at this point to use to taste for gelling as the jam reaches completion.)

Place the chopped mangoes into a heavy-bottomed saucepan along with the sugar and citrus juice. Be sure that they only come 1/3rd of the way up the pot at this point to prevent overflows when the mango jam is cooking.

Bring the ingredients to a boil over medium-high heat, stirring frequently to prevent scorching or overflows.

Boil hard for about 20 minutes until the jam begins to gel. Test for gelling on a plate that's been chilled in the freezer, or use an instant-ready thermometer to check for gel temperature (220 degrees F, or slightly lower at higher altitudes).