I left Thailand without getting a stamp on my single-entry retirement visa. I understand that I must now re-apply and complete the whole process again. Can I re-apply in Bangkok--given that the Thai embassy in Bangkok has all of my relevant docments? I'm a US citizen.

I checked in at Phuket Town, where my visa was stamped. Does that make a difference?

Thank you so much for your help.

You should obtain a non immigrant O visa prior to your return and then you can do the retirement extension at any immigration office two months after entry - if you do not want to do the O-A visa from the USA again. The money will have to be in a Thai account if done here.

FYI: Thailand does not have an Embassy in Bangkok - that is for foreign countries to have. It is immigration hqs.