I am leaving Thailand for a couple of months.  When I return I will apply for a new passport at the US embassy.  

1.  I will be in Bangkok with some visiting family and thought I would just go to the embassy and apply while we are there.  I understand I need to make an appointment. 

2.  If I do this will they allow me to keep my old passport while traveling around Thailand with my family?

3.  Is the processing time 4 to 5 weeks for the new passport?

4.  I was told I would have to send the old passport to the Embassy once I receive my new passport. I assume I can keep it long enough to have all visa stamps etc. moved to my new passport at the immigration office.

Thanks for the help.

You almost certainly don’t need to go to the embassy to renew your passport, you can check in the attached link.

 

https://th.usembassy.gov/u-s-citizen-services/passports/adult-passport-renew/

1.  You are highly encouraged to renew your passport by mail at the link given (where you will also find the answers to some of your questions):

https://th.usembassy.gov/u-s-citizen-services/passports/adult-passport-renew/

2.  No.  You submit your old passport and will be without a passport until your new passport is ready at which time you will be sent your new passport along with your old passport (invalidated by having a hole punched through it).    You should make copies of your old passport, both paper copies and pictures which can be stored on your smartphone, before applying.  Make copies of all passport pages relevant to being in Thailand:  photo id page, latest visa or extension of stay, latest entry into Thailand stamp, and arrival/departure card TM-6 (if any), TM-30 address registration receipt (if any), 90-Day Report receipt/next appointment due date (if any).  If you have a 90-Day Report coming due shortly you may wish to postpone your application for a new passport until you have completed your 90-Day Report (which may be done up to 15 days before its due date).  You don't need a physical copy of your passport in order to do a 90-Day Report using the online system.  

Copies of your passport may allow you to travel around Thailand.  It's best to have other photo id, such as a Thai driving license, a pink Thai ID card, or foreign driving license.  It's not ideal to travel without your passport.

3.  Processing time is currently given as up to 4 to 5 weeks by mail (it's the same for in person).

4.  You get your new passport and old (invalidated) passport returned to you by mail.  You take both of these to your local immigration office and can have the relevant data (including old passport number, original visa information, current extension of stay, original entry into Thailand stamp, and latest entry into Thailand stamp) transferred from the old to the new passport.  If you have a non-expired visa in your old passport it will not be transferred but you will have to travel with both the new and old (invalidated passport containing the valid visa) with you when traveling.

I recently renewed using the online procedure. It was difficult to find the correct starting page - there are some old ones that can lead you down a rabbit hole. I needed to setup a Login.gov account to make the payment. I needed a cashier's check from Bangkok Bank (any bank is okay) to pay for the return postage. All in all, it was relatively simple. I was really impressed that I got the new passport in 2 weeks. Good service overall.

39 minutes ago, timendres said:

I recently renewed using the online procedure. It was difficult to find the correct starting page - there are some old ones that can lead you down a rabbit hole. I needed to setup a Login.gov account to make the payment. I needed a cashier's check from Bangkok Bank (any bank is okay) to pay for the return postage. All in all, it was relatively simple. I was really impressed that I got the new passport in 2 weeks. Good service overall.

A Google search (us passport thailand) returns the correct link as the first search result:

https://th.usembassy.gov/u-s-citizen-services/passports/adult-passport-renew/

but the embassy site is a large one and I guess it is possible to get off on the wrong foot.

It's correct that you need a cashier's check (bank draft) from a Thai bank to pay for the return envelope and postage but the instructions on what to ask the Thai bank for are very good and in Thai for you to take to the bank with you.  You can also use a separate cashier's check to pay for the passport book.  You do need to use a cashier's check to pay for a passport card, if you choose to get one.  So, you could have used two Thai bank drafts to pay for your order instead of using the Login.gov site to make a payment for the passport.  I found that site/process of paying online a bit cumbersome but I wanted to try it out and it does work.  It should be easier once you are registered there, but probably in 10 years the process of paying will be even easier but different. ???? It is indeed impressive that they can deliver your new and old passports to your Thai address by Thai Post EMS in 2 weeks (although now they estimate that process to take 4-5 weeks).  It's a good service, indeed.