Ollie Watkins gave the Bees a third-minute lead before Jason Knight equalized on 29 minutes. Said Benrahma put the hosts back in front four minutes after halftime and doubled Brentford's lead in the 64th minute.
The visitors are now three points behind joint leaders Leeds United and West Bromwich Albion while Derby's play-off hopes took a hit, the defeat dropping the Rams to 10th in the table, three points from sixth.
Story of the game
The vaunted "BMW" trio of Benrahma, Bryan Mbeumo and Watkins produced the opening goal as Josh Dasilva and Benrahma combined for some lovely interplay with the former picking out Mbeumo in the Derby penalty area. His shot cannons off the post and came back out to Watkins, who fired home the rebound.
Brentford should have doubled their lead when Dasilva picked out an overlapping Rico Henry who crossed for Mbeumo. His shot was heading for the top corner, but Matt Clarke made a timely intervention with his head.
Derby began growing into the game and Max Bird tried his luck from distance only to see his shot go high over the crossbar. Chris Martin lashed one at Bees goalkeeper David Raya, who tipped it in the air and caught it on the way down. Martin was instrumental in setting up the hosts' equalizer.
The midfielder profited from a lucky ricochet off of defender Christian Norgaard and found Wayne Rooney roaming free. His shot was blocked, but the former England international re-gathered in the penalty area and found Knight, who put it past Raya to make it 1-1.
Derby continued to be on the front foot as Jordan Sibley shot from the near post after a drinks break, but his shot was well over. Soon after, the Rams' defense kept the scoreline level, the wall blocking a free kick and Rooney getting in the way of Dasilva's shot. Ben Hamer saved Benrahma's curler to keep the contest level heading into halftime.
With the help of Hamer, Brentford re-took the lead four minutes into the second half. After Emiliano Marcondes picked out Benrahma on the left, the Bees' talisman cut in, fired at the Derby shot-stopper with pace and he let the ball slip under him to hand the advantage back to the visitors.
The promotion-chasing Londoners went looking for a third, Dasilva testing Hamer, who this time pushed the shot wide of his post. Benrahma hit the Derby wall with a free kick and Norgaard went wide with his effort. All of these chances finally led to Brentford's third.
Marcondes spread play out, found Benrahma, who cut inside two Derby defenders and rifled an unstoppable shot into the top left corner from 25 yards out to give the Bees a two-goal advantage. The hosts did get more chances as Sibley volleyed a free kick over and Raya saved from Knight, who was set up by Rooney and Martin.
Both teams had shouts for a penalty with Benrahma playing in Watkins, who was clattered into by George Evans and Henry battling with Tom Lawrence in the air at the other end. Tariq Fosu tried his luck only to see Hamer come up with the stop while Joel Valencia having his shot deflected and Watkins finding the side netting.
By then, the matter was settled and Brentford looked for all the world like a team that will plying their trade in the top flight of English football for the first time ever next season.
Man of the Match: Said Benrahma
It couldn't be anyone else as he scored two mesmerizing goals and was involved in almost every other key Brentford move. With his power, pace, vision and creativity, he'll be a superstar of the highest order should the Bees achieve promotion to the Premier League.
Brentford could have had five or six goals
If it wasn't for some timely blocks and deflections, the scoreline could have been worse for Derby as Brentford were sharp, incisive and precise in their build-up play with Dasilva being one of the keys in setting up the Bees for quality chances. The fact that they were still hunting for goals late on when the outcome was decided shows you the mindset Thomas Frank's club has.
Wayne Rooney was effectively shut down
While the focus is rightfully on the Brentford offense, their defense shouldn't be overlooked as they neutralized the Derby skipper and largely kept him from creating any significant or quality chances. While he was involved in the Rams' goal, he had little say in the match otherwise thanks to the Bees' backline.