In a microwave-safe mug, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until well combined.
Add the milk, vegetable oil, fresh lemon juice, lemon zest, and vanilla extract to the dry ingredients in the mug.
Mix thoroughly with a fork or small whisk until the batter is smooth and free of lumps.
Microwave the mug on high power for 1 minute. Check the cake for doneness by inserting a toothpick into the center; it should come out clean or with a few moist crumbs.
If the cake is not fully cooked, microwave in additional 10-second intervals until done, being careful not to overcook.
Allow the mug cake to cool for 2 minutes before enjoying.