This script uses the REST API GitHub to transfer active (open) issues from a Mantis bug database CSV export into a GitHub Issue database. See the code for the columns required. Note that although the existing issue ID from Mantis is encoded into the GitHub issue text, the GitHub issue will be created with an entirely new (non-configurable) issue ID.
The GitHub issues will be created using the account that is used to access the REST API, so it is recommended that a new GitHub user account be created explicitly for this purpose. You will then need to create a GitHub access token for that account with permission to create issues. That token should be stored in a file someplace: there is a configuration variable in the script for setting that file's location.
Second, if you are importing more than about 150 tickets, expect the process to be interrupted by GitHub's rate limiter. The code will shut itself down when it detects that case: you will need to wait some length of time (I went with one hour), then restart from the last ticket attempted. I did not automate that restart process. We imported about 800 tickets, which took most of the day, running in batches of ~150 each hour.
There are some configuration variables in the source code related to Mantis-to-GitHub username mapping: note that if any of the mapped ticket assignees do not have the appropriate GitHub permissions, the importer will fail with a 403: Unprocessible Entity error. It will print out the name of the failed assignment, which you will either need to remove from the list, or give assignment permissions to in your repo.
It is intended that the Mantis instance is kept alive for some significant time after the migration, in particular to support attachements. As of this writing there is no public, documented GitHub REST API for uploading files, so this script does not migrate attachments, instead opting to provide a back-link to the original Mantis ticket.
An auxilliary script is provided to create Mantis database entries linking back to the GitHub issue. This script is intended to be run on the server with the Mantis instance on it, and has configuration variables at the top. It requires the BBCodePlus Mantis plugin (or you can modify the script to not use the BBCode tags for the URL).
The FreeCAD project used this importer to migrate our Mantis database: you can see the results starting at FreeCAD/FreeCAD#5538, and the original issues at https://tracker.freecad.org