Daniel Nardiello was amongast the scorers against his former club as an injury hit Millers overpowered their visitors to move just a point off the automatic promotion places.
Ben Pringle got the ball rolling after only 60 seconds as he picked up the ball in midfield and advanced to fire a drive past the diving Arthur Krysiak.
And the home side were out of sight by the half hour mark, as a further two goals in as many minutes doubled their tally.
Nardiello was on the end of a neat one two with Pringle down the left to slot in the second, before Michael O'Connor beat Krysiak for a third time with a deflected effort from 25 yards.
The one way traffic continued early in the second period as defender Craig Morgan got in on the act by powerfully volleying in from close range after Nardiello nodded a free-kick back across the area.
Nardiello just missed out on netting his second before City, who to their credit continued to try and play good football, threatened at the other end as loanee Lawson D'Ath shot over.
Danny Coles grabbed a consolation with 12 minutes left as he headed in Aaron Davies' corner, but the game had long gone.
Their fourth loss on the bounce sees Exeter drop out of the play-off zone at the expense of Bradford, who beat Bristol Rovers.