Bleach is the answer, but rinse like you mean it.

Bleach kills mold and mildew in a cooler, but only if you rinse it well and dry it completely afterward.

Mix about a cup of bleach into a gallon of water. Scrub with a soft brush or sponge — nothing abrasive, because deep scratches in the plastic give mold a place to hide next time. Pay extra attention to the corners and the rubber seal. Let the solution sit for ten minutes or so, then dump it out and rinse the whole thing until you can’t smell bleach anymore.

The key step is drying. Mold needs moisture, so leave the lid open in the sun for a day or two. If you’re storing it long-term, keep the lid cracked and make sure it’s bone dry before you close it. That’s what prevents the whole problem from coming back.

It’s a simple job and totally worth doing right. A swamp-smelling cooler ruins pretty much every trip.