It was the third successive defeat for the Hammers and their new manager Manuel Pellegrini, whose side made a promising start at the Emirates stadium when Austria striker Arnautovic rifled in a fine low shot from 20 metres.
West Ham's defensive weaknesses, however, continued to haunt them as Monreal equalised with a fierce shot from eight metres past two defenders on the line after goalkeeper Lukasz Fabianski had parried Hector Bellerin's cross into the defender's path.
Fabianski and his Arsenal counterpart made a series of good saves before more slack defending by the visitors allowed Arsenal to turn the match on its head, with Issa Diop committing a costly double error.
The centre-back's poorly lofted clearance rebounded to halftime substitute Alexandre Lacazette and his vicious cut-back from a tight angle cannoned off Diop, past Fabianski.
Welbeck, a late substitute for Pierre-Emerick Aubameyang, sealed Arsenal's win in stoppage time after Bellerin's fine run and cross gave him time and space to score from close range.
Spaniard Emery, who took over at Arsenal in the summer from long-serving manager Arsene Wenger after a two-year stint at French champions Paris Saint-Germain, was happy with the outcome but acknowledged there was plenty of room for improvement.
"The three points were very important," Emery told Sky Sports.
"Today I saw good things, but also things (to) improve -- more transition. For the second half we were speaking in the dressing room (of the need) to help more inside with two midfielders.
"We need the balance. In the first half we suffered more than we want."
INDIVIDUAL QUALITY
Former Arsenal keeper Fabianski was upbeat despite the defeat.
"It's all about keeping belief, because when you look at our squad there is a lot of quality individually," he told the BBC.
"There is still a lot of work to be done, but from what I have seen today we are going in the right direction."
(Reporting by Zoran Milosavljevic; Editing by Ed Osmond, Neville Dalton)
Share
