A goalless first period was punctuated by an injury to Arsenal midfielder Mohamed Elneny, who was stretchered off the field with an ankle problem.
After the break, the Gunners were quickly in front, as Nacho Monreal volleyed home a sweet strike from a corner. West Ham battled back well, though, and Marko Arnautovic hammered in a left-footed equaliser in the 64th minute.
Arsenal produced a late rally to pick up the three points, though, after Aaron Ramsey's cross found its way into the net with eight minute's remaining, restoring his side's lead. Lacazette then added the gloss to the scoreline, as he grabbed a brace to continue his hot streak in front of goal.
Arsenal Start Slowly
While there was a pre-match buzz and some serenading for manager Arsene Wenger following the announcement he will leave the club at the end of the campaign, a much-changed Arsenal team struggled to get going in the first half.
With Mesut Ozil absent because of illness and Pierre-Emerick Aubameyang only on the bench, there was little spark on show at the Emirates Stadium, with West Ham able to contain their opponents for long spells.
Mark Ogden of ESPN FC commented on the lack of quality on show from the Arsenal players:
It was the visitors who had the better opportunities, with Arnautovic a particular pest to the Arsenal back line. He stung David Ospina's palms at one point after scampering into the box from the left flank.
At the other end, Danny Welbeck flashed wide for the Gunners from the edge of the area and also missed the target with a header.
Elneny Injury Blow
The main incident of the first half was an unfortunate one, as Elneny was stretchered off for the Gunners after a rash challenge from Mark Noble.
As Bleacher Report's Sam Tighe noted, to pick up what appeared to be a serious injury with a UEFA Europa League semi-final and FIFA World Cup to come, the timing is cruel for the Egypt international:
Elneny was replaced by Ainsley Maitland-Niles on the stroke of half-time. Elneny's injury will leave Wenger with a big decision to make with Thursday's clash against Atletico Madrid in mind.
Gunners Pegged Back
After a slow first period, Arsenal started the second period with extra intent and got in front after some poor defending allowed Monreal to fire home his fifth goal of the campaign.
However, Arsenal were unable to make their dominance count, and inspired by Manuel Lanzini's introduction, West Ham got themselves level through the irrepressible Arnautovic.
As Squawka Football noted, he's enjoying an excellent campaign in his new role as a centre-forward:
The Austria international lashed home a brilliant left-footed finish after the ball dropped to him in the penalty area. The West Ham supporters went wild, while the majority of the Emirates Stadium lamented more poor defending from their players.
Luck and Lacazette See Gunners Home
It took a stroke of fortune for the Gunners to get back in front in the 82nd minute.
Ramsey's cross wasn't dealt with by West Ham defender Declan Rice, who inexplicably ducked under the ball. Goalkeeper Joe Hart was wrong-footed as a result, and the cross drifted into the far corner.
Per the Daily Mail's Lee Clayton, it was a rare error from a young centre-back who's been playing well as of late:
With West Ham piling forward in search of an equaliser, Arsenal and Lacazette capitalised on the extra space afforded to them, with the striker bagging two goals in the space of four minutes.
The France international's first came at the end of an excellent passing move, slotting home at the near post from a tight angle. Lacazette then showed his predatory instincts again, making some room for himself in the penalty area with some neat footwork before finishing calmly.