Place 5 black tea bags, a few sprigs of mint, and 1 cup of sugar into a large, heatproof pitcher or vessel that can hold at least 9 cups of liquid.
Pour 4 cups of freshly boiled water over the tea bags, mint, and sugar in the pitcher.
Let it steep for 10 minutes so the tea and mint infuse thoroughly, and the sugar fully dissolves.
Remove the tea bags and mint sprigs. Press the tea bags gently against the pitcher to extract the maximum flavor before discarding.
Stir in 2/3 cup of fresh lemon juice to brighten and balance the flavor.
Pour in 4 cups of cold water to cool down and balance the strength of the brewed tea.
Refrigerate the mixture for 1 to 2 hours, or until thoroughly chilled before serving.
When ready to serve, pour the iced tea over ice cubes in glasses. Garnish with fresh mint sprigs or lemon slices, if desired.