Spray the slow cooker with non-stick spray to prevent sticking.
Place the diced sweet potatoes in the slow cooker. Add the brown sugar, maple syrup, sliced butter, pumpkin pie spice, and salt. Stir to ensure the sweet potatoes are evenly coated with the mixture.
Cover the slow cooker with the lid and set it on high for 4 hours.
Five minutes before serving, remove the lid and evenly distribute the mini marshmallows over the top of the sweet potatoes.
Replace the lid and allow the marshmallows to melt for 3 to 5 minutes.
Once the marshmallows are gooey and lightly browned, serve the sweet potatoes immediately, scooping out the delicious mixture into individual servings.