Yes—you can absolutely grow mint in an urban planter box without it taking over everything, but you need to treat mint as a contained guest, not a free-roaming garden plant. Mint spreads mainly by underground runners, called rhizomes. In open soil, those runners travel sideways and pop up new plants. In a planter box, you can control that spread if you block the runners, keep the roots isolated, and stay on top of pruning.
The first question is what kind of planter box you have. A closed-bottom box on a balcony, roof terrace, or windowsill is much easier than an open-bottom bed that touches garden soil. If the box has no barrier between its soil and the ground, mint will eventually find a way out and become a problem. If it sits on concrete or pavers, the risk is lower, but it can still spread across the box and crowd out other herbs.
The simplest reliable method is to grow mint in its own pot and sink that pot into the urban planter box. Choose a pot that is at least 8 to 12 inches wide, with drainage holes, and bury it so the rim sits slightly above the soil line. This creates a physical wall around the roots. The mint looks like it is part of the planter display, but its runners cannot easily escape into the surrounding soil. If you want several mint varieties, use separate pots. Peppermint, spearmint, apple mint, and chocolate mint all spread, so mixing them in one container usually turns into a tangled competition.
If you prefer to plant mint directly in the planter box, use a sturdy root barrier or a dedicated compartment. Make sure the barrier goes all the way down and has no gaps. Even then, mint can send stems over the top and root where they touch moist soil. That is why I recommend keeping the mint's soil level slightly lower than the surrounding soil, or leaving a clear dry gap around the mint patch. Check the edges every week or two, especially in summer.
Regular harvesting is your best maintenance tool. Cutting mint often keeps it bushy and limits runner production. Snip sprigs for tea, salads, cocktails, and sauces. Remove any runner that crosses the barrier or tries to escape the pot. If flowers appear and you do not want seedlings, trim them off, although most mint spread in containers is vegetative rather than from seed. Do not put pulled runners in compost unless your compost gets very hot; they can survive and regrow.
Mint in a container also needs the right growing conditions. Use a quality potting mix that drains well but holds some moisture. Mint likes partial sun, though in hot climates afternoon shade is welcome. Container mint dries out faster than garden mint, so water when the top inch of soil feels dry. Feed lightly every few weeks during the growing season. In winter, mint may die back. In cold areas, move the planter box to a protected spot or mulch it so the roots do not freeze solid.
So, can an urban planter box contain mint? Yes—if you use a pot-in-pot system, a root barrier, or a sealed dedicated compartment. Without those controls, mint will likely take over the box and maybe beyond. With them, you get fresh mint on demand and a planter that still looks intentional. Treat mint like a vigorous but manageable tenant, and it will reward you instead of running the whole building.