Saido Berahino’s double ended his own nine-game drought as he made it 20 goals for the season for the Baggies.
Chris Brunt added a third to compound Chelsea’s miserable night in the West Midlands as their 16-game unbeaten league run ended.
And Fabregas’ first-half dismissal will go down as one of the most mindless in the history of the Premier League.
The midfielder saw red after kicking the ball against the side of Brunt’s head as referee Mike Jones took control of a clash between Gareth McAuley and Diego Costa.
The champions were afforded a guard of honour as they came onto the pitch but the well-drilled Baggies refused to stand on ceremony to claim a deserved win.
Chelsea rarely threatened, even before Fabregas walked, although Loic Remy hit the post in the second half.
They had been boosted by the return of Costa after a six-week lay off with a hamstring problem while Nemanja Matic also returned with Gary Cahill, in after Kurt Zouma’s season-ending knee injury.
Berahino, Callum McManaman and James Morrison were recalled by Albion and McManaman nearly fired them into a fifth-minute lead.
The winger ghosted past Filipe Luis only for Thibaut Courtois’ excellent reaction stop to deny him a first goal for the club.
Boaz Myhill then claimed Fabregas’ shot after the midfielder had wriggled through, before the Baggies took the lead in spectacular style in the ninth minute.
Berahino received the ball 25 yards out and allowed it to roll across him before arching a first-time drive into the bottom corner.
Matic drilled wide in reply but the organised Baggies comfortably kept Chelsea at bay, with the flat atmosphere underlining the lack of importance of the game.
But a routine night suddenly flared up in curious fashion when Fabregas was sent off after 29 minutes.
It began when Costa and McAuley grappled in the area, with the Chelsea striker booked after the pair squared up to each other.
And while referee Jones was calming the situation down Fabregas – from 20 yards away – kicked the ball towards the group and against the side of Brunt’s head.
Darren Fletcher then squared up to the midfielder before Fabregas was sent off, his first red card since January 2006.
It was stupid from the former Barcelona star but gave the game much-needed bite. Fletcher was booked for his part in the incident.
Courtois denied Brunt after Berahino’s quick break and Myhill saved Luis’ free-kick after Jonas Olsson chopped down Costa.
The anonymous Remy dragged a shot wide at the end of the first half but Chelsea were 2-0 behind before they could even blink after the break.
Just 26 seconds after the restart John Terry scythed down Berahino in the area and the striker beat Courtois from the spot.
Remy thumped the post from 20 yards but it was only a brief threat from the visitors and Brunt sealed Albion’s victory on the hour.
Chelsea switched off from a short corner and the midfielder was allowed to run into the box and lash past Courtois at his near post.
Jose Mourinho looked on disgusted and his mood failed to improve when Myhill saved well from Matic with 17 minutes left.
The quiet Eden Hazard fired over late on but there was nothing he could do to stop Chelsea slipping to just their third league defeat of the season.