Alexis Sanchez's scrambled extra-time winner secured Arsenal an FA Cup final date with Chelsea - and ensured Manchester City manager Pep Guardiola will end a season without a trophy for the first time in his coaching career.
Monreal made amends with the equaliser 11 minutes later as he drilled in Alex Oxlade-Chamberlain's cross at the far post - but City were left nursing a serious sense of injustice after this disappointing defeat.
Wenger persisted with an unfamiliar three-man defensive system comprising youngster Rob Holding, Gabriel and Laurent Koscielny and set up his team to deliver an uncharacteristically stubborn performance.
Guardiola, arguably football's most celebrated coach, was brought to Manchester City to lift them to another level - and on that basis his first season without a trophy in a glittering managerial career will be regarded by many as a failure.
He must now address the problems that have undermined City this season, particularly uncertainty over the goalkeeper position, where his decision to replace Joe Hart with Claudio Bravo has been unsuccessful, and also sort out an uncertain and ageing defence.