Clark County District Attorney Lyndsey Boon Brunette tells WSAW-TV Wisconsin Rapids Mayor Zach Vruwink will not be charged.
Vruwink said he posted the photo in February, but took it down immediately after learning it was a crime.
Showing a completed ballot to someone else is a felony in Wisconsin, punishable by up to 18 months in prison and a $10,000 fine. The issue came up during the 2012 recall elections when Democratic Party Chairman Mike Tate posted a photo of his ballot online.
The district attorney didn't provide a reason to the station.
Vruwink had no comment when contacted by The Associated Press via email Tuesday.