Arsenal went into the game looking to leapfrog Tony Pulis’ side in the table and they did so thanks to Lacazette's brace.
With all the fanfare pre-match about Gareth Barry, who broke the Premier League appearance record by playing his 663rd game, the veteran midfielder said that he was only focussed on getting the win.
Unfortunately for the former England man, Arsenal hadn't read the script and condemned the evergreen midfielder to defeat on his big day.
The hosts started the game well, looking to dominate and control proceedings, with Alexis Sanchez seeing an effort from a free kick drop just wide.
However, West Brom fired a warning shot after just eight minutes as they almost took a shock lead.
Man of the moment Barry set Jay Rodriguez away down the left, with the England international chopping back inside Mustafi and firing low at goal, only to see Cech turn his effort onto the post.
Rodriguez could well have been awarded a penalty for the challenge by Mustafi but referee Bobby Madley said nothing doing.
Forays forward in the opening exchanges from the visitors were at a premium though and Arsenal soon took the lead.
Sanchez was given another chance from a dead ball after Evans crunched into Elneny on the edge of the box. The Chilean clipped the free kick over the wall and back off the bar, but French international Alexandre Lacazette was on hand to head home from the rebound to give the hosts the advantage.
Arsenal looked very much in control for the 10 minutes that followed, spraying the ball about at will but without really creating much, though Nacho Monreal did force Nyom and Hegazy into a last-ditch challenge.
Sanchez, who was at the heart of much of Arsenal's best play, wanted a penalty when he clashed with Krychowiak in the box following a corner but the officials were not convinced.
At that point if appeared that Arsenal would cruise to victory, but West Brom ended the half strongly.
Rodriguez saw a header cleared off the line by Monreal after great work down the left from Krychowiak before the Pole dispossessed Elneny and fed the former Burnley man who fizzed an effort just wide from range.
Krychowiak was a growing influence as the half went on and could have got his name on the scoresheet after 42 minutes but he couldn’t quite stretch enough to get a touch on Hal Robson-Kanu’s flicked header.
Despite a spell of pressure from the ‘Baggies’, Arsenal held out to go into the break ahead.
Unfortunately for the visitors, they were unable to mount a similar period of pressure in the second half, with Arsenal again taking control from the outset.
Lacazette almost doubled his tally for the evening when Elneny laid Kolasinac’s low cross from the left into his patch but Kieran Gibbs was on hand to get in a block.
Mustafi saw a nice flick from a corner easily gathered by Foster, but it was only a matter of time before Arsenal would make things safe.
In the end, it came gift-wrapped, with Nyom barging Ramsey over on the byline to give the hosts a penalty.
Lacazette stepped up and made no mistake from the spot, firing low into the bottom right corner, despite Foster having guessed right, for his and Arsenal’s second of the night.
That goal meant Lacazette has scored as many goals as West Brom in the Premier League this season and the Frenchman almost overtook the ‘Baggies’ minutes later when he plucked the ball out of the air and rounded Foster, only for his finishing touch to desert him and cause him to lash the ball over the bar.
The game was over at that point, with Pulis’ substitutions failing to turn the game or even stem the tide.
Arsenal could have added a third late on as substitute Mesut Ozil was denied by the legs of Foster on the counter but a third would have been harsh on the visitors.
The victory moved Arsenal up to 7th in the Premier League table, with West Brom dropping down to 12th.
Arsene Wenger's attentions will now turn to the Europa League clash against BATE Borisov on Thursday.