You can leave an expired guild from the Guild section of the Mabinogi website. You must be logged into your account with your Mabinogi character on it. Some information may be cached on this page, so if you experience any issues, try deleting your cookies or browsing data, or try using a different web browser.
Follow the steps below:
(Before doing so, please make sure your character is currently logged off while doing the steps.)
1. Under 'Community Tab' select 'Guilds'
2. Select the appropriate character beside 'MY INFO' and select that character's -Registered Guild-.
3. You will be routed to 'MY GUILD INFO' section. You will see [Leave The Guild] on the same row, please click it.