I live in downtown DC and have planted mandevillas in my backyard. I wonder if I can overwinter them the same way I do my banana plants. In the fall I cut the banana plants down and cover with up to a foot of mulch and then cover with plastic and secure with rocks to prevent wind blow-away. Uncover in the spring. Banana plants reach 12-15 feet by end of summer. Would the same kind of treatment maybe work with my mandevillas?
Regardless of how much mulch you use, mandevilla should not over-winter outdoors. If you want to preserve the plant, cut it as little as possible and pot it up. Store the plant under your house or in the garage for the winter, or indoors.
When you move it back outside it is not going to look perky. Spring is the time to cut it back. That is also when you can replant it in the ground if you choose, or grow it in a container. Start fertilizing and watering when you move it back outside and it should recover. If you choose to protect it for the winter, you need to lift and store before a killing frost. If you plan to pot it up and bring it inside your house for the winter, you need to do this by early October.
Of course, you can always experiment. Since you are in the city, perhaps you can find a micro-climate (sunny sheltered spot) that never freezes, in which case maybe you can manage to get your mandevilla through the winter alive. But, they will not take even heavy frost.