Cut a slit in each of the sweet potatoes, lengthwise down the middle. Add the sweet potatoes to the base of the slow cooker and add about 1 cup of water. Enough to cover the bottom with 1/4 inch. Set on low and cook for 8 hours.
Carefully remove the sweet potatoes and let cool for a few minutes before removing the skin. Scoop out all the soft flesh into a mixing bowl. Add butter (you don't need a ton because they are already very soft) and cinnamon and mix together.
Sprinkle the brown sugar over the top. Sprinkle the pecans over the top. (Both of these are optional! I generally don't serve 'fancy sweet potatoes' for regular weeknight dinners. I would add them to a holiday side dish, however!)
Serve Slow Cooker Mashed Sweet Potatoes as side dish.