Can you play Mystery Manor offline?

In general yes you can, but you will not be able to participate in clan events and competitions, and you cannot watch video ads for energy, coins etc.

Your progress will be saved on your device but you will need to get online to make sure it is backed up to the server so that you don't lose any progress if you have a problem with your device.

For more questions for Mystery Manor: hidden objects check out the answers page where you can search or ask your own question.