Paul Peschisolido made a dream debut for Derby as he capped an inspired performance by scoring the winning goal in the second half for George Burley's strugglers.
The Rams had dominated the opening 45 minutes but were unable to make their superiority count against a gritty Rotherham outfit.
Burley's side, who have not moved out of the relegation zone because of wins for Walsall and Nottingham Forest, bolstered their squad this week by signing defender Jeff Kenna from Birmingham and striker Peschisolido from Sheffield United.
Both were named in the Rams' starting line-up.
Rotherham, who could still be sucked back into the relegation battle, also needed a win to put more distance between themselves and the bottom three.
Boss Ronnie Moore included Phil Gilchrist in his starting XI after signing him on an initial month's loan from West Brom, even though the defender's last first-team game for the Baggies was on Boxing Day.
Rotherham's Julien Baudet cleared an early corner from Candido Costa and Chris Swailes then blocked a piledriver from Tom Huddlestone as the home side started strongly.
Peschisolido, keen to make a quick impression on his new employers, then sent a fifth-minute shot wide as Derby continued to dominate the early exchanges.
Rotherham's attacking forays were limited in the opening 10 minutes and a Darren Garner cross was easily cut out by Derby keeper Lee Grant on one of the few occasions they looked threatening early on.
The home side continued to look much more impressive with Rotherham keeper Mike Pollitt having to punch clear from a Costa free-kick and Peschisolido having another goalbound effort blocked.
Derby were forced to make a change on the half-hour with Paul Boertien replacing the injured Michael Johnson.
Rotherham's Paul Warne did try his luck from distance, but his effort 10 minutes before the interval was too high.
The home side continued to push for the opener in the second half with Peschisolido, full of running and testing the visiting defence, impressing on his debut.
Lee Holmes sent a shot wide of the target for Derby, who continued to press Rotherham back with Peschisolido and Huddlestone both having efforts on goal in quick succession.
Peschisolido then brought a fine save out of Pollitt after 57 minutes as the visitors somehow clung on.
But it proved a brief respite as Derby deservedly took the lead on the hour through Peschisolido.
Richard Jackson received the ball on the right and his cross found the former Sheffield United striker who lashed the ball home on the rebound after his initial effort had been kept out by the over-worked Pollitt.
Peschisolido went agonisingly close to adding a second just seven minutes later, but his left-footed curler was superbly saved by Pollitt.
Pollitt distinguished himself again in injury-time with another superb save to deny Huddlestone.
Teams
Derby Grant, Kenna, Mawene, Johnson (Boertien 31), Jackson,
Costa (Holmes 45), Taylor, Huddlestone, Osman,
Paul Peschisolido (Whelan 90), Paul Peschisolido (Whelan 90),
Tudgay.
Subs Not Used: Oakes, Labarthe Tome.
Goals: Paul Peschisolido 61.
Rotherham Pollitt, Stockdale, Gilchrist,
Richard Barker (Proctor 68), Hurst, Garner (Hoskins 77), Minto,
Baudet (Mullin 70), Swailes, Butler, Warne.
Subs Not Used: Montgomery, Daws.
Booked: Butler.
Att: 21,741
Ref: A Leake (Lancashire).
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