An improved mailer folding apparatus that forms mailers from mailer blanks. The mailer folding apparatus comprises a transport assembly and one or more folding assemblies. The transport assembly moves mailer blanks and/or mailers through the mailer folding apparatus. The one or more folding assemblies may each comprise a folding system and a carriage system. The folding system may selectively allow for the folding of a mailer blank into a mailer. A carriage of the carriage system may move the folding system longitudinally between different positions relative to the transport assembly. An engagement system separate from the carriage system or integrated into the carriage system may engage and disengage the folding system with the transport system for selectively feeding a mailer blank through the folding system, or disengage the folding system for allowing a mailer blank or folded mailer to bypass the folding system.