Patrick Bamford claimed just his second goal for the club when he turned on the edge of the box before sending a shot into the bottom corner after just seven minutes.
Boro then extended their advantage through Yanic Wildschut who was making his first start for the club.
The winger showed a great bit of skill to open up the chance and he fired in a left-footed strike past Adam Collin in the Millers' goal.
Wildschut raced down the right-hand side moments after the sending-off, but Kike spurned the cut-back by firing over when well placed, before wasting a similar opportunity shortly after.
Wildschut almost capped off his display with an assist but his whipped cross just evaded Jelle Vossen who had earlier replaced Bamford.
Lee Tomlin's solo effort late on claimed the points for the visitors who move up to second behind leaders Watford.