Chelsea had gotten off to a strong start as they dominated the game from the kick-off, and they were soon in the lead after Juan Mata scored the first goal of the Benitez era after 13 minutes, ending a 5-hour goal drought for the blues by smartly finishing after fine work from Fernando Torres.
It was to be Torres' only meaningful contribution to the game however, as hopes that Benitez would rejuvenate him continued to look optimistic, the striker typically floundering up front, and when presented with an almost identical opportunity to tee up Juan Mata, his cut-back was overhit and the chance was gone.
Chelsea almost paid immediately when Kevin Nolan collected the ball in the area and swivelled to fire the ball into the net, but the goal was ruled out for a push on Branislav Ivanovic, a controversial decision after replays showed there was very little in it.
West Ham had struggled up until half-time and were perhaps fortunate, despite their disallowed goal, to go in level, but Sam Allardyce responded assertively with his half-time changes. James Tomkins had looked lost in midfield and Gary O'Neil had contributed little, and both were withdrawn for Mohamed Diame and Matt Taylor respectively.
Diame in particular gave West Ham a renewed bite in midfield, and allowed the Hammers to come back into the game. Their goal finally came after 63 minutes, Carlton Cole rising over Ivanovic to head home a Matt Jarvis cross. Chelsea were incensed and demanded that the goal was disallowed for a foul - and had more than a case to suggest so - but perhaps justice had been done in the light of the earlier decision.
Minutes later, Chelsea almost immediately regained their advantage, Juan Mata smacking the ball off the post from a free-kick, but they were unable to stop a resurgent West Ham from putting them under yet more pressure. Matt Jarvis surged into the area, but Petr Cech was alert to tip his shot around the post, and from the corner, Ashley Cole was called into action to clear Winston Reid's header off the line.
With Chelsea rocking, the Hammers finally found their breakthrough, Carlton Cole setting up Mohamed Diame to smash the ball into the net with a low ferocious drive. It was a richly-deserved goal for the midfielder, who had changed the game since coming on and was a surprise not to start after his excellent form since signing for the East Londoners.
That left Chelsea with little time to salvage something from the game - although it got even worse for the Blues. Ashley Cole tamely surrendered possession to present Modibo Maiga with a fine opportunity. After unselfishly setting up Matt Jarvis, Cech made the save from the former Wolves winger, but Maiga had followed up to smash home the rebound.
What started out brightly had turned into a disaster for Chelsea, with Rafael Benitez now the only manager of the Abramovich era to fail to win his first three games, and the first for the club since Gianluca Vialli all the way back in 1998. With the fans not on his side to begin with, performances and results will have done nothing to convince them they were in the wrong. His first two games were flat, dour affairs, but this - a meek surrender after a strong start - presents a different shade of incompetence.
Things are looking grim for the Blues unless Benitez can do something special in his next few games, although full credit should go to West Ham for the victory. Allardyce probably made two mistakes in his team selection, but he had the conviction to change them at half-time and was rewarded with a fine second-half performance. The Hammers looked out of place in the Europa League spots, but they would look similarly odd in the relegation places.