Hold the mangoes standing and with a knife, cut from the top down on one side of the pit to separate flesh. Repeat on other side. Using a spoon, scoop flesh and coarsely chop. Scrape flesh from pits, if any. Discard pits.
In a pot over low heat, combine mango, sugar, lemon juice and pectin. Cook, stirring regularly and mashing with back of spoon, until sugar is dissolved. Bring to a boil.
Increase heat to medium and continue to cook, skimming foam that floats to top, until mixture thickens. To check for doneness, use the "parting the sea" technique by transferring a teaspoonful onto a small dish and refrigerate for about 1 minute. Run your finger in the middle of jam. If it remains open and does not flow back together, it is ready.
Transfer mango jam to jars while hot. Allow to cool before sealing completely and store in refrigerator.