Milltown is located at 43°25'23" North, 97°48'5" West (43.423333, -97.801389).
According to the United States Census Bureau, the township has a total area of 0.43 mi2 (1.12 km2). 0.43 mi2 (1.11 km2) of it is land and 0.00 mi2 (0.01 km2) of it is covered by water. The total area is 0.45% water.
Milltown is located in the Central Standard Time (CST) timezone, GMT-06:00. It observes Daylight Saving Time (DST).