Kenny Jackett had shocked the club by resigning as manager after just five games last Monday and Paul Warne was handed the reins on a temporary basis.
But the fitness coach could only look on as a goal in each half at Burton Albion condemned the division's bottom club to a 14th league defeat of the season.
Midfielder Tom Adeyemi turned in Izzy Brown's cross deep into time added on to reduce the arrears but it was too little too late.
Rotherham, fired up by Warne's threat to substitute anyone not giving their all after 20 minutes, started well and should have taken the lead in the 11th minute.
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The Brown-Jon Taylor link-up had already threatened before Brown played in the scampering winger on the right who should have done better than see his cross-goal shot deflected for a corner.
It started to go wrong when old boy Chris O'Grady, not for the first or last time, was too strong in his battle with Dominic Ball four minutes later and the ball fell for Jackson Irvine to steer home.
In an ugly, scrappy half, the home side had most of the pressure and it took a flying save from Lewis Price to keep out an O'Grady piledriver.
Rotherham's only response was a header wide when Brown failed to make the most of Danny Ward's delivery from the left.
Little changed after the break and Matty Palmer fired in from 20 yards in the 62nd minute to double Albion's advantage.
Goalkeeper Lee Camp missed out injured for the Millers and Warne dropped midfielder Joe Newell to the bench and brought in winger Anthony Forde for his first start since August.
The Millers, with just seven points from 18 matches, are now 13 adrift of safety at the foot of the table.
Burton Albion (3-5-2): McLaughlin; Brayford, Mousinho, Turner; Akins, Irvine, Palmer, Naylor (Choudhury 75), Dyer; O'Grady (Flanagan 86), Ward (Miller 79). Subs not used: Bywater, Williamson, McCrory, Harness.
Rotherham United (4-4-1-1): Price; Fisher, Ball, Wood, Mattock; Taylor (Yates 67), Adeyemi, Vaulks, Forde (Newell 67); Brown; Ward. Subs not used: Bilboe, Fry, Kelly, Blackstock, Belaid.
Goals: Irvine 15, Palmer 62 (Burton); Adeyemi 90+3 (Rotherham).
Referee: Christopher Kavanagh (Lancashire).