Map Milwaukee, the City of Milwaukee's online Geographic Information System, has maps of historic districts.
Go to the Map Milwaukee "Zoning Application" website
Click on the right facing arrow at top left to access map layers.
Click on the box labeled "Layer Theme" and select Historic Preservation.
From here you can select the two types of historic districts that appear as polygons and the individually designated properties that appear as stars.
If you have zoomed in close enough to see parcel lines, you can use the "Identify" tool on the left side of the window to determine the address and taxkeys of parcels located in designated districts.
Additional information and help with Map Milwaukee is available on its main page here: http://city.milwaukee.gov/mapmilwaukee.
Note: As of September 2017, the system does not work with the Chrome browser. Please use Internet Explorer/Edge, Firefox, or Safari.