Odion Ighalo continued his superb run of scoring form with another two goals while strike partner Troy Deeney also netted as the Hornets remained six points behind leaders Derby.
Rotherham came into the game on the back of conceding five at Wolves on Saturday and their defensive misfortune continued in the 17th minute when they gifted Watford the lead.
Kari Arnason was under no pressure when he received the ball around 25 yards from his goal but he slipped and Ighalo pounced immediately, rounding the stranded Adam Collin to stroke in his 16th goal of the season.
But for the athleticism of the Rotherham keeper, the Hornets would have doubled their lead by the 25th minute as the home side were denied three times in quick succession.
Collin produced good saves to keep out shots from Almen Abdi and Deeney, but the Millers’ stopper’s third save was the pick, plunging to his left to keep out Joel Ekstrand’s back-heeled near-post flick from a corner.
It was Ekstrand who nearly allowed Rotherham a route back into the contest when his lapse gave Matt Derbyshire a clear run into the penalty area from the left, but Heurelho Gomes proved equal to his effort.
The Millers kept the pressure on though, winning three corners in quick succession and from the third Craig Morgan really should have done better when he was left unmarked, but he headed straight at Gomes.
Despite leading, Watford had not got out of first gear for much of the first half but within 11 second-half minutes they had effectively put the game to bed.
Rotherham’s problems started when Arnason was booked for bringing down Daniel Tozser around 30 yards out.
The Hungarian took the 54th-minute free-kick himself, blasting an effort straight at Collin which the Millers keeper got behind but was unable to hold. Ekstrand reacted the quickest and squared for Deeney to finish from close range.
And two became three for the Hornets inside two minutes as Miguel Layun was set up for a left-footed shot which Collin was unable to hold and the ball the broke loose for Ighalo to net his second of the evening in straightforward fashion.
Rotherham did have chances to reduce the deficit towards the end as Derbyshire saw a couple of efforts deflected wide, but they could also have been on the end of a heavier defeat.
Tommie Hoban and Almen Abdi both put good opportunities over the bar, Matej Vydra forced a good save out of Collin and, in injury time, Abdi hit the post when he seemed almost certain to add a fourth.