I suspect that the mint has become potbound so pot full of roots and no compost to hold nutrients and moisturel. You could lift the mint from the pot and teese out a few roots and repot them. Better still might be best to buy some potted mint from the supermarket of garden centre and start afresh.

Is it still mint? Does it taste & smell of mint?

Could be the thing eventually starved and what you see is minty minus soil nutrients

Some options,
Do as Jimmy suggests
Remove from pot & start over in fresh soil
Give it a darn good liquid feed of any general use fertiliser

That is odd isn’t it? Could it be that your mint died and a weed grew in its place?but it does look similar to mint. I am flummoxed!

it looks as if it has been starved of nutrients. re pot or give it a good foliar feed. that is if it still smells of mint.

alternatively have a look at the roots are they fleshyish and white? it also looks a bit like enchanters nightshade but it will have taken a year or so to look this full.