I see on the Thai Embassy website in DC that to get a Thai passport you have to come in person. Does this mean any Thai citizen in need of a new passport must go to Washington DC?

 

Also, is it possible to leave the US on an expired Thai passport and get new Thai passport in Thailand? Will that cause problems?

If you go to the various Thai Consulates websites that are located in the US you will see that they all issue passports to Thai Citizens, so you don't have to go only to Washington D.C.in order to renew a Thai Passport

 

Here is where you will find a list of them with web site addresses:

 

http://www.thaiembassy.org/main/

 

There have been reports of Thais returning to Thailand with an expired passport without problems but I would be concerned about a foreign airline letting you board without a valid passport,  so I would check with the airline first to be safe

 

 

Don't know where you will be but Los Angeles also has a Thai consulate.

I don't know when you'll be traveling, but the Thai consulate has been going around the country renewing passports.  They were at the big temple here in Dallas for 5 days last month renewing passports.  If you decide to go that route, you can register online for an appointment.  My wife didn't do that and we were there all day, just to have them tell us no because my wife had more than a year of validity left on her passport.  They call it Mobile Service.  Here's a link to it, but it's all in Thai:

 

http://www.thaiembdc.org/dcdp/?q=both/Mobile_Service