Some travel mugs leak.
Others do not.
It all comes down to the lid.
And also to what you mean by “leak.”
If you are looking for a tumbler that can be sloshed around during a rough car ride, without splashing your hot coffee everywhere, then most will fit the bill.
Any mug that comes with a lid will prevent splashing. The main lid styles are a silicone lid that slips over the rim of the cup and a plastic lid that goes inside the rim and usually seals with silicone rings wound the outside.
Both of these types form a good seal with the cup, but both have a hole in the lid for drinking. The plastic types usually allow to to close the hole by flipping or sliding the lid opening shut.
If you need a tumbler that can be tipped over and still not have any liquid seep out, then these types of lids will not work.
What Is The Best Leak-Proof Travel Mug?
The only type of lid that is completely spill-proof is one that screws on and that does not have an opening you can drink through. These are they types of lids you usually find on a thermos bottle.
There are some travel mugs that have screw-on lids, however, even some made from ceramic. Check out these, for example.
That said, most completely spill-proof mugs are made from metal and some from plastic.