In a microwave-safe mug (at least 12 oz capacity), mash the ripe banana with a fork until smooth.
Add the all-purpose flour, granulated sugar, baking powder, ground cinnamon, and salt to the mug. Stir well to combine.
Add the vegetable oil, milk, and vanilla extract to the mug. Mix thoroughly until a smooth batter forms.
Microwave the mug on high for 90 to 120 seconds. Check for doneness by inserting a toothpick into the center; it should come out clean or with a few moist crumbs.
If not fully cooked, microwave in additional 10-second increments until done, being careful not to overcook.
Allow the banana bread to cool for 2 minutes before eating directly from the mug.