The City of Portland and the Peaks Island Council have worked together to install a much needed public toilet in an area away from Down Front.
The toilet won’t be obvious to the casual hiker, but you can find it at Trott Littlejohn Park, on Florida and Island Avenue, just under 1 mile from the ferry terminal.  Google maps link to Trott Littlejohn
While visiting, check out Trott Littlejohn and its community garden.
For other toilet locations see https://peaksisland.info/FAQ