An impressive Benjani hat-trick ended Portsmouth's six-match goal drought at Fratton Park against a Derby side who now appear certain to be relegated.
Benjani notched his second hat-trick of the season at Fratton Park
The Rams had the better of an explosive start with 19-year-old defender Lewin Nyatanga stabbing home on five minutes.
But Benjani's powerful shot and precise header gave Pompey the lead just before half-time and he coolly finished off a swift counter-attack on 55 minutes.
Midfielder Lassana Diarra was quietly influential on his Portmouth debut.
For a side mathematically on course to become the worst ever in the Premier League, Derby were full of belief and triggered a frenetic opening.
With all five of Paul Jewell's January transfer window signings in the starting line-up, the Rams looked refreshed and ready.
On five minutes former Portsmouth winger Laurent Robert whipped in a trademark free-kick to the back post and, after the ball was headed back across goal, Pompey keeper David James could only push Emanuel Villa's weak attempt against the bar.
With the ball loose, Nyatanga prodded home from six yards for his first Premier League goal.
Derby could have nicked another goal seconds later but James atoned for his error with a smart save when one-on-one from Kenny Miller just before Portsmouth striker Kevin Nugent volleyed over from eight yards.
Though the pace remained frenetic, both sides looked poor in possession with Portsmouth particularly nervy at the back without club captain Sol Campbell.
But a four-minute spell just before half-time changed the course of the match.
Pompey's first goal arrived when Sylvian Distin drifted an intelligent ball into the penalty area for Nugent and Pedro Mendes to work an opening for Benjani.
With his back to goal, the Zimbabwean international wasted no time in turning and smashing the ball into the top corner from eight yards.
Seconds later, he rose unmarked to head past Derby keeper Lewis Price from Hermann Hreidarsson's cross and Pompey were walking down the tunnel with an unlikely lead.
Within ten minutes of the restart, Pompey had put the result beyond question to get their European aspirations back on track.
Benjani and Mendes played a one-two as they raced clear on the counter-attack and, with calculated precision, the African striker steered the ball past Price's dive into the far corner of the goal.
Portsmouth's Croatian international Nico Kranjcar, impressive throughout, went close with a couple of decent efforts as the game drifted to its conclusion.
Derby are now 13 points adrift of safety with just 16 games left to play.
Portsmouth boss Harry Redknapp:
"It was a good win for us. We got off to a bad start but I think the crowd played a big part today. It's easy to get frustrated but they really helped us.
"It's unusual for us, finding goals difficult to come by. But they were three wonderful goals by Benjani. We always make a lot of chances here and it was three good points in the end.
"Derby need a miracle to get out but they started well and passed the ball well. But we came back strong.
"Nico Krancjar did some fantastic things with the ball out on the left for us. We'll just keep going, I don't know about Europe."
Derby manager Paul Jewell:
"We started very well and could have gone two goals up but some of the defending from us beggars belief.
"I keep on saying the same things over and over again and it doesn't take a rocket scientist to work out that's why we are where we are.
"I don't know what happens to some people when they get out there. It's going to be difficult, I'm not going to say that we are going to stay up.
"Hopefully the FA Cup can give us a ray of light. It's a tough time but the fans were terrific again today."
Portsmouth: James, Johnson, Pamarot, Distin, Hreidarsson, Lauren, Diarra (Hughes 81), Pedro Mendes, Kranjcar, Mwaruwari (Mvuemba 89), Nugent.
Subs Not Used: Begovic, Aubey, Songo'o.
Goals: Mwaruwari 38, 42, 55.
Derby: Price, Mills (Lewis 32), Todd, Moore, Nyatanga, Fagan (Teale 68), Savage, Ghaly, Robert (Barnes 56), Villa, Miller.
Subs Not Used: Hinchliffe, Earnshaw.
Booked: Fagan, Villa.
Goals: Nyatanga 4.
Ref: Mike Dean (Wirral).
BBC Sport Player Rater man of the match: Portsmouth's Benjani 8.47 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.