The 27-year-old 'Teenage Dream' hitmaker reportedly went on a date with the guitarist just a day before she met Brand at the MTV Video Music Awards in 2009, and later fell head over heels over the funnyman in Los Angeles, reported Femalefirst online.
Perry is reportedly now in a relationship with Mayer after her divorce with Brand was finalised.
"A couple of friends had set Katy and John up on this date. They had an amazing time and were planning to do it again. Then she went to the VMAs and fell head over heels for Russell," a source said.
"But it must have always felt like unfinished business with her and John, and they'd occasionally text each other while she was with Russell. It's no surprise to friends they have hooked up now her divorce has been finalised," the source added.