Cesar Azpilicueta's first-half own goal and a late strike from Bryan Mbeumo is what separated the two sides in this West London derby, as it was another underwhelming performance from the Blues.
It marks back-to-back victories at the Bridge for Brentford since they were promoted, after Thomas Frank's side won 4-1 here last season.
The win also ends a six-game winless run for the Bees, as their European hopes are given a minor lift.
With the likes of Kai Havertz, Mason Mount and Reece James out injured, and Joao Felix left on the bench, Chelsea started the evening with a starting XI that had scored just 10 goals between them all season.
Their tally of 30 Premier League goals so far this season is the lowest they have registered in a top-flight season for 99 years, and their performance this evening explained why.
In a first period devoid of clear-cut chances, David Raya was hardly tested by a backtracking header from Thiago Silva, and a dipping effort from Enzo Fernandez.
Brentford bided their time and frustrated Chelsea before taking the lead from a set piece towards the end of the first half.
Mathias Jensen's delivery got an important touch off Mathias Jorgensen, which then struck an unaware Azpilicueta and evaded Kepa Arrizabalaga, who stood no chance in the Chelsea goal to put Brentford 1-0 up.
Boos rang around Stamford Bridge on the sound of the referee's half-time whistle as once more Chelsea never looked like scoring.
Pierre-Emerick Aubameyang was introduced after the break along with Mykhaylo Mudryk, and the Gabonese forward was among the action in the early stages of the second 45.
In just his second appearance for almost three months, Aubameyang had three effort in five second-half minutes, the latter of which was the most promising opening, but his tame, deflected effort was comfortable for Raya.
Chelsea continued to cause no problems to a stubborn Brentford rearguard as the visitors were more than happy to wait and hit on the counter.
That tactic worked a treat for Frank and his side, as it led to a second goal with just over 10 minutes remaining.
Mbeumo timed his run to his perfection to keep himself onside to receive a through ball from Shandon Baptiste, ran at Wesley Fofana and beat the French defender before firing beyond Kepa at his near post, via a deflection off Silva.
The goal brought about a mass exodus among the home support, while there were jubilant scenes in the away end behind the goal where Mbeumo netted.
Brentford and their supporters thoroughly enjoyed the final few moments of the game, claiming bragging rights in West London by easily seeing out the game and securing a 2-0 success.
Lampard has now lost all five games since taking caretaker charge after the sacking of Graham Potter, with the club winless in eight.
A first bottom-half finish looks on the cards for Chelsea for the first time since the 1995-96 season, as they sit six points behind Fulham in 10th.
Brentford have moved to within six points of the top seven, and they will now have one eye on Tottenham Hotspur's result against Manchester United tomorrow evening.
Chelsea have another daunting prospect ahead, facing Arsenal next up on Tuesday, while Brentford welcome relegation-threatened Nottingham Forest to West London on Saturday.