iOS
If you're using MindMeister Basic, you can close your account directly within the iOS app My Account page. Read this article to find out how.
If you subscribed to MindMeister Pro or Business through the app store, you must cancel your subscription through your iOS settings before closing your account. Find out how to do this here. Then close your account on iOS, as described here.
Android
If you're using MindMeister Basic, you cannot close your account on the Android app. Visit the web app and follow the guide here to close your account.
If you subscribed to MindMeister Pro or Business through the Google PlayStore, you must cancel your subscription through your settings before closing your account. Find out how to do this here. Then close your account via the web app, as explained here.