Released from kennels for the first time since 6 January because of Ian Wright's hamstring injury, he popped in a goal within two minutes and went on to harry West Ham to such good effect that an addition to their five successive Premiership victories rarely looked likely.
Hartson's belligerent attitude even extended to an early challenge on Julian Dicks severe enough to earn a booking. Arsenal are not Dicks's favourite opponents: sent off at Highbury when West Ham lost in September, he passed up an opportunity to equalise at an important stage of the game yesterday by missing a second-half penalty.
Arsenal, even without Tony Adams and Steve Bould, defended with their old cussedness to record a fifth win in six visits to Upton Park. On a pitch too sticky for their passing game, West Ham relied on Iain Dowie's aerial ability. His header two minutes from time appeared to have saved the match, but a linesman's flag said otherwise.
West Ham lacked width, especially down the right, which John Harkes could not provide on his own from full-back as Danny Williamson and Michael Hughes were preoccupied by the elusive Paul Merson. "We gave them a terrible, diabolical goal," Harry Redknapp, the West Ham manager, said. "Julian was suffering from double vision when he took the penalty, his left foot is normally like radar."
Harkes, who was responsible for Arsenal taking the lead, could not blame the pitch for the casual nudge back towards his goalkeeper that Merson collected, eluding Steve Potts and picking out Hartson.
The home team's response did not bear the stamp of a side in form, such troublesome moments as there were for David Seaman tending to stem from mishit crosses rather than anything more purposeful.
Despite Arsenal's two cup defeats and only one victory in six previous games, there was not a hint of anything resembling panic in their manager Bruce Rioch's approach to the game. Hartson's inclusion was the only change following Wednesday's Coca-Cola Cup semi-final disappointment, with Martin Keown, Andy Linighan and Steve Morrow retained in the centre of defence.
Not until half-time was anything altered, David Platt coming off the substitutes' bench to replace David Hillier, who had strained a calf. His initial duties were defensive as West Ham piled forward in the rain.
The penalty was awarded, without significant protest, as Dowie played the ball in for Tony Cottee, who was bundled over by Keown. Seaman fell to his left to parry Dicks' kick and held the follow-up from Harkes, who soon made way for Dani, but this was not a day, or a pitch, for subtle Latin skills.