In a medium bowl, whisk together the maple syrup, brown sugar, and Dijon mustard until smooth to make the glaze.
Take the ham out of the package and discard the glaze packet. Use a towel to pat the ham dry. If you bought an uncut ham, use a sharp knife to cut a crisscross pattern into it. (If you bought a spiral-cut ham, don't add any extra cuts.)
Place the ham in the bottom of a slow cooker. Pour the glaze all over the top and sides of the ham.
Cover and cook on LOW for 8 hours or on HIGH for 4 hours. Use a pastry brush to coat the ham in the glaze and juice in the slow cooker every hour or so during the cooking time.
Carefully remove the ham from the slow cooker and transfer it to a serving platter. Cut into slices (if needed) and serve the extra sauce from the slow cooker on the side to pour over the ham slices.
Notes
City hams are the type of ham you'll find in the meat department of pretty much any US grocery store. They usually come spiral cut or whole/not cut in a package with a glaze packet. These hams are precooked, so all you have to do is flavor and reheat the ham.