Place potatoes into the slow cooker and add the broth. Cook on High for 2 hours. Toss the potatoes halfway through to keep them moist.
Use a slotted spoon to transfer the potatoes to a large mixing bowl. Reserve the extra broth. Using a potato masher, mash the potatoes until smooth. Add in the margarine, milk, and 1/2 cup of the reserved broth. Mix until desired consistency. Salt as needed (this will largely depend on if you used low sodium chicken broth or not).
NOTE: I like to save the remaining reserved broth and use it to my Challah Stuffing recipe