To get the Pokemon TCG Pocket Mew card, as mentioned above, you need to complete the Kanto Pokedex, which means getting at least one of every of the original 150 Pokemon in the game. Once you do so, an option will come up in your "missions" tab to complete the quest and reward you with the Mew card itself, like any other mission reward.

Pokemon TCG Pocket's secret missions are the most difficult in the game, and players deserve better for their efforts than what they're given.

All of the Secret Missions inCG Pocket listed, including the cards you need, and their booster packs, for the Pokémon Pocket Secret Missions.