Calum Chambers grabbed our opener, while Alexandre Lacazette, Nicolas Pepe and Folarin Balogun were also on target.
There was also a first glimpse of Albert Sambi Lokonga for the Gunners, with the 21-year-old completing 45 minutes in central midfield alongside first Thomas Partey, and then second-half substitute Ainsley Maitland-Niles.
Alex Mitchell's close-range finish from a late Millwall corner was a minor blotch on the performance, but overall Mikel Arteta will be pleased with his side's professional pre-season display following the potential disruption that could have been caused by the cancellation of the US tour.
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