Palace got better as the game progressed and the Eagles deserved their late consolation through Luka Milivojevic, but manager Roy Hodgson will be desperately disappointed to have suffered just a second defeat in 13 matches in such circumstances.
Arsenal went into the contest short of confidence and without Alexis Sanchez - who is close to joining Manchester United - but the belief came rushing back in a 19-minute spell at the start of the game.
The opening goal came from the returning Monreal, who was perfectly placed at the back post to head into the net from close range after meeting Granit Xhaka's corner.
The home side quickly doubled their lead when Monreal turned provider, with the Spaniard reaching the byline before giving Iwobi the chance to convert from no more than six yards out.
Palace looked lively when they went on the attack but there was a sense that they could concede each time that they came under pressure and Arsenal's third goal came after just 13 minutes, with Koscielny bundling the ball home after Monreal had been found at the back post from a corner.
The fourth goal was the best of the lot, with a team move ending with Mesut Ozil flicking the ball into the path of Lacazette, who coolly slotted the ball into the bottom corner from 16 yards out to end his seven-week goal drought.
To Palace's credit, they improved as the first half progressed but aside from a weak effort from Bakary Sako and a deflected strike from Wilfried Zaha which was tipped wide by Petr Cech, the visitors did not look like troubling the scorers.
In the first 10 minutes after the restart, it was Palace who made most of the running but only a shot on the turn from Martin Kelly came close to getting the visitors back into the match.
However, Arsenal should have scored a fifth before the hour mark when Iwobi was released down the left flank, only to shoot straight at Wayne Hennessey from a tight angle rather than square the ball to Lacazette for a tap in.
Back up the other end, Christian Benteke was given the ball on the right-hand side of the penalty area, but the Belgian forward drilled his resulting effort from 14 yards too close to Cech.
Arsenal looked more likely to net a fifth as the game moved closer to a conclusion with Mohamed Elneny seeing a well-struck effort kept out by Hennessey at his near post.
With 12 minutes left, though, Palace got their consolation goal when Milivojevic reacted fastest inside the penalty area to volley into the far corner from eight yards out.
The final stages of the game were played out without incident - aside from an injury to Yohan Cabaye - but Arsenal were content to see the match out to get back into contention for a place in the top four of the table.
ARSENAL (4-5-1): Cech; Bellerin, Mustafi, Koscielny, Monreal (Maitland-Niles 34'); Elneny, Xhaka; Ozil (Nelson (72'), Wilshere, Iwobi (Kolasinac 82'); Lacazette
CRYSTAL PALACE (4-4-2): Hennessey; Fosu-Mensah, Tomkins, Kelly, Van Aanholt; McCarthur, Cabaye (Riedewald 90'), Milivojevic, Zaha; Sako, Benteke