The letter doesn't specify an office, though Johnston's manager, Tank Jones, has said Johnston's interested in a run for mayor or City Council. He did not immediately return a message Monday.
The soonest he could appear on the ballot is in October 2011. He filed too late to run in this fall's City Council races, and the mayor's office doesn't come up for a vote again until next year.
Earlier this month, Jones confirmed that Johnston planned to run for office as part of a reality TV show.
Johnston has had a prickly relationship with the Palin family since the birth of his son, Tripp. He and Bristol Palin recently broke off a short-lived engagement.
The filing was first reported by TMZ.com.
Important warning about e-mails purporting to be from the adn.com staff.