Both stars have been romantically linked to Jennifer Aniston in recent months and Mayer officially dated the actress last year (08), but their unplanned meeting in Los Angeles was natural and friendly - and observed by a journalist.
Mayer reportedly spotted the Scottish actor dining at the Chateau Marmont hotel while he was being interviewed for Details.
He broke off from his chat to walk over and greet Butler, who seemed delighted to see Mayer.
The reporter, Steven Daly, writes, "For 20 minutes, the rest of the world disappears as the pair huddle together in what appears to be an intense discussion."
Upon his return, Mayer insists the two men "talked about New York," adding, "We're in a bit of the same position of being branded womanisers."
Both Mayer and Butler have denied recent romantic links to Aniston.