While there are ATMs around the festival (check the maps!), there is a great way to avoid carrying cards and cash around the whole time: Bonnaroo Cashless! Bonnaroo Cashless is the easiest way for you to pay for food, drinks, merch and more with the tap of a wrist. Cashless is secure, free, and easy to use. You’ll simply link a credit card to your wrisband when you register your wristband before the festival (you can always update the info by registering again on the same page).
It's good to have some cash around just in case, but be sure to lock it in your car when not using it.