Where can you dispose of old lawn mower gas?

Most municipalities have some type of hazardous waste recycling facilities where you can dispose of old gasoline.

You could probably also take it to an auto mechanic. Depending on the amount, they may dispose of it for you for free.

Assuming you are talking about a small amount of fuel, you could put it in your vehicle's fuel tank. Just make sure the tank is at least 3/4 full when adding the old fuel. The small amount of old gas will mix with the newer gas in the car and won't hardly make a difference.