It's expected that MantisHub users will need to access their service when they are on the go and away from their desktops. Rather than producing an app, MantisHub provides an optimized mobile interface using your mobile browser. So when you receive email notifications or updates via your chat integration you can simply click on the issue links to read further or action.
When accessing your MantisHub on your mobile, you have access via a default responsive mobile UI. This is the same UI you get on the desktop browser but adapted to smaller screen. It has the full product functionality powered by the core service.
The responsive UI is designed to work well on any mobile device that has a modern HTML 5 browser. And you can create a mobile web app on both Android and iPhone to sit on your home page for easy access.
In addition to the above, you will find some native apps in the App Store or Play Store that are developed by 3rd party developers by search for "Mantis Bug Tracker". This would typically need to be installed or purchased via the store by each of your users.