Fill a 16 ounce copper mug 2/3 of the way full with ice cubes.
Pour the tequila over the ice.
Add the fresh lime juice to the mug.
Pour in the ginger beer and gently stir to combine.
Add garnishes of your choice such as limes and mint, then serve immediately.
Notes
Ginger beer is a sweet fizzy drink. You can buy it in small bottles in the mixers area near the alcohol or sometimes it is in the soda area. If you can't find it, you can use ginger ale although the flavor won't be quite the same.
This recipe makes one strong drink. You can easily adjust the number of servings in the recipe card to fit your occasion and the ingredient amounts will scale up.
If you prefer to shake your mule, you can place the tequila and lime juice in a cocktail shaker. Shake the ingredients together, then pour them over the ice in the mug and add the ginger beer.