A couple of days ago my roommate fertilized his mint with a geranium feed, but unfortunately, the mint does not seem to have appreciated it - the leaves became brown and mostly fell: see the picture below (lower mint; the feed is getting to the upper one).
This is clearly over fertilization which could mean death! This mint just might make it. That plant and pot needs to be totally soaked and drained like right now. Get rid of the babies. Then remove the main part of the mint plant to get OUT of the chemistry laden soil. Shake the roots gently to get rid of as much soil as possible. A pot 8" in diameter and 6" high will work fine for now. Don't upgrade again until you see roots coming out of the bottom drainage hole.
Use ONLY fresh, sterilized potting medium without fertilizer or water holding gimmicks.
Make sure the bottom of your pot is off the surface of the saucer or the patio floor or whatever. Airspace between the bottom of the pot and the surface is almost critical for great drainage. I use 1/4" pieces of broken tile.
When the soil in the new pot is watered thoroughly, pick that pot up and FEEL its weight/heft. Do not water again until you pick that pot up again and it is DEFINITELY lighter.
There are potting feet that are cute but expensive.
Mint is a major survivor and it should pull out of this poisoning from too much fertilizer if you get the poison away from the roots of the plant like now.
Here is my little ditty concerning fertilizer:
Less is Best, More is Death, None is Dumb.
You are seeing what happens when there is a little too much fertilizer.
Allow this plant to settle and recuperate. When this plant demonstrates it is alive and thriving...THEN wait a few MONTHS use Osmocote 14-14-14 at HALF the directed amount.
If you're going to fertilize mint at all, it needs to be something with an NPK of no more than 7-7-7, in other words, a general purpose, basic fertilizer. The fertilizer that was used, if it was meant for 'geraniums' (presumably meaning Pelargoniums rather than true Geranium) would likely have had a much higher K (potassium) reading, because that encourages flowering. Fertilizers come with a varied range of NPK, formulated for use on different types of plants, so what's appropriate for long flowering plants won't be appropriate to use on plants grown primarily for their foliage.
Mint will grow in poor soil and does not really need much fertilizer - in containers, they quickly outgrow the space available and need to be turned out, split and replanted at least annually to ensure continuing healthy growth.
Luckily one of the plants seems to be relatively healthy still, so it would be wise to turn the contents of the box out, remove as much of the soil as possible, get some new potting soil and replant the healthiest mint into that, watering in well. You may need to cut it back if further browning takes place.