Romain Vincelot's first-half header settled a fiery Yorkshire derby and means the Bantams have been beaten just once in their opening eight league games.
Their goal came in the 21st minute when Vincelot was allowed the freedom of the Millers' six-yard box to head home Tony McMahon's cross.
Rotherham were showing nothing of the form that saw them hammer Walsall 5-1 in midweek, though they should have equalised five minutes later. Jon Taylor's first effort was blocked by Colin Doyle and then his second was heroically cleared off the line by Matt Kilgallon.
Bradford should have gone into half-time with a 2-0 advantage as Dominic Poleon went through on goal but Rotherham goalkeeper Marek Rodak bravely blocked
The Millers improved after the restart and fashioned several chances to get back into the game as Kieffer Moore's shot was saved by Doyle while Nathaniel Knight-Percival's brilliant block denied Lee Frecklington's goalbound shot.
It was not to be their afternoon, though, and a disappointing outing was capped off with Joe Mattock's red card deep into injury time for two bookings.