I doubt its going to turn bad, unless the garden area was used before for waste, or had lots of chemicals placed there.

If it is peppermint or spearmint you probably don't want it for the main mint in your mojito. I prefer applemint for mine. I find that the fuzzy leafed mints seem to go better with rum than the smoother leafed ones which I put in tea.

I agree with ksrogers about it being bad unless a toxic waste dump or a member is spray happy you should be fine to use it. But with all things try a small amount first to see how you like it and how your body reacts to the mint.

Spearmint has more oval shaped leaves and makes a great warm drink with a little honey or not. It was all wild at one time.

I'm not sure what kind of mint it is. It's a good example of why mint should be planted in pots - if we let it go i think it'd take over all 95 plots in our community garden!

Here's some pics of it:

And although this is blurry it's a good example of how it's taken over!

Odd ... I can't help on the foreground picture since they don't look like either my peppermint or spearmint but the colored flowers and leaves in the background look very much like gill / alehoof. A mint family and very invasive by runners, even under other mints.

The foreground is the mint i'm talking about.

What does it smell like--peppermint, spearmint, or something else?

There are many kinds of mint, and this may just be one that those that have posted so far are not familiar with--or it could be a hybrid of two different mints, such as orange mint, which I believe has a round and smooth leaf, and apple mint, which I think is a "fuzzier" mint.

As long as it smells like mint and has a square stem, it should be OK to use, provided no one has sprayed it with poison. Rinse it off just to be safe, and use it in hot teas, at least at first. Mint has some medicinal attributes, and therefore be moderate in your use of it unless you do a bit of research first.