You can renew your passport at any time and may renew your passport by mail while you are in Thailand, please see the detailed instructions here https://th.usembassy.gov/u-s-citizen-services/passports/adult-passport-renew/. Passport renewal process may take up to 6 to 8 weeks from when we receive the complete application package.
Please note that expedited service is unavailable when applying for U.S. passport overseas.
However, we could return your current valid passport while the new one is processing. You are required to do the following when preparing your application package:
Pay for two Thailand Post Courier Pass Back Envelopes here https://www.thailandpostmart.com/product/1013460000929/ (one is to return your current passport, another is for mailing your new passport)
If the page is in Thai, please use languages/translate function at the top right corner of the page,
complete all required information, see *asterisks,
Please ensure to provide the applicant’s full name and accurate returning address when purchasing the envelope.
Write on application checklist as “Need to have current valid passport returned.”
You will be asked to mail in your current passport for cancellation as soon as a new passport is ready. Your new passport will not be mailed until we have cancelled your current passport.
Regards,
American Citizens Services Unit
U.S. Embassy Bangkok