Long scored the first and set up a second from Peter Odemwingie to give the Baggies a victory which cements their best start to a league season in 35 years and extends the visitors' winless Premier League run to four matches.
The result gave West Brom boss Steve Clarke a famous triumph over his former club, while Roberto Di Matteo tasted defeat at the same ground where Andre Villas-Boas' Blues reign ended last term.
Di Matteo fielded a makeshift side with one eye firmly on Tuesday’s crucial Champions League clash away at Juventus.
Captain John Terry missed out having injured himself against Liverpool, with Juan Mata, Oscar, Ashley Cole, Ramires and Branislav Ivanovic all rested.
West Brom, meanwhile, fielded Boaz Myhill in goal in place of the injured Ben Foster and Long up front in the absence of Romelu Lukaku, with the Belgian ineligible against his parent club.
Chelsea started brightly and, on seven minutes, Victor Moses’ pull-back found Ryan Bertrand, whose shot was cleared off the line by Liam Ridgewell, with Jonas Olsson blocking Eden Hazard’s rebound.
But against the run of play it was the hosts who took the lead three minutes later with their first meaningful attack.
Zoltan Gera pulled the ball back to James Morrison, whose pinpoint cross found Long and, having evaded a statuesque David Luiz, the Irishman nodded in from close range.
The Baggies continued to look dangerous going forward, and on 35 minutes Long shrugged off Cahill before curling a low effort just wide of Petr Cech’s post.
It was a miss they rued four minutes later when Cesar Azpilicueta’s hanging cross was met by Hazard, and his back-post header deflected off Billy Jones and past Myhill.
Sturridge and Moses both threatened as Chelsea finished the half strongly, but wayward finishing ensured both sides went in level at the break.
The visitors began the second half as they had ended the first, and on 47 minutes Moses’ driving run and cross found Sturridge, who tested Myhill with a fierce strike on the turn.
But three minutes later West Brom edged ahead again, with Long the provider this time.
After a sweeping counterattack, the Irishman curled a superb ball in from the right for Odemwingie, who nodded his fourth Premier League goal of the season at the near post.
Chelsea pushed on again, and a minute later only a sliding block from Olsson denied Sturridge a simple tap-in from another dangerous Azpilicueta delivery.
Di Matteo introduced Mata and Oscar for Oriol Romeu and the ineffective Fernando Torres just after the hour, with Sturridge assuming his preferred role as central striker.
The changes almost had the desired effect when, with 12 minutes left, the Spaniard sent Sturridge clear, but the youngster’s firm strike was well saved by Myhill from close range.
Four minutes from time Sturridge missed another gilt-edged chance, but West Brom held on.