Mills distributed seven assists and Duncan totaled 11 points with 13 rebounds as the Spurs claimed a seventh win in eight games.
Marco Belinelli added 14 points for San Antonio, which dominated the boards and drained 11-of-18 3-point attempts.
The Bucks hit just 37.5 percent shooting from the field in dropping a ninth straight.
Brandon Knight led Milwaukee with 21 points. John Henson added 10 with 11 rebounds.