Mash avocados in a bowl with a fork until slightly chunky. Fold tomatillos, mango, onion, serrano chile pepper, cilantro, and mint through the avocado; add lemon juice and gently mix. Season the guacamole with salt.
Lay a piece of plastic wrap directly on the surface of the guacamole so it doesn't discolor and refrigerate until chilled, at least 1 hour.