Peel and dice mangoes into small pieces and set aside.
In chilled bowl, use a handheld mixer to whip heavy cream to soft peaks. Continue to whip while slowing adding sweetened condensed milk. Whip to firm peaks.
In a 8x8 or 9x9 pan arrange a single layer of graham crackers. Break the graham crackers into smaller pieces if needed to fit the shape of the dish.
Spread a generous layer of the sweetened whipped cream over the graham crackers.
Distribute about 1 cup of diced mango over whipped cream. Place a layer of graham crackers over mangoes. Repeat twice by adding another layer of whipped cream followed by mangoes.
Cover the cake with plastic wrap or aluminum foil and refrigerate for at least 4 hours, or overnight if possible.