Combine mashed potatoes, egg yolks, 1/4 cup sugar, butter, milk, salt, and sherry; mix well. Spoon mixture into a greased 9- inch pie plate. Make a well in the center of mixture; smooth edges with a spatula.
Beat egg whites (at room temperature) until soft peaks form. Gradually add remaining sugar, 1 tablespoon at a time, beating until stiff peaks form.
Using a decorator bag and a medium-size tip, fill well with meringue, and make dollops around top of sweet potato mixture. Bake at 325° for 15 minutes or until the meringue is lightly browned.