A crowd of over 1,000, along with City manager Sven-Goran Eriksson and assistant Hans Backe, braved bitterly cold conditions to see the visitors take an early lead through Suriya Domtaisong on 11 minutes, while Mpenza went close with a pair of chances near the end of the half.
Substitute Adam Clayton spurned City's best opportunity of the second half, and Thailand went further ahead with 20 minutes left when Sarayoot Chaikamdee lobbed Hartley.
City Chairman Dr Thaksin Shinawatra attended the game, posing for photographs with fans and handing out raffle prizes at half time.
Blues reserves coach Ian Miller told City's website: "Technically Thailand were very good, and they asked questions of our young defenders that they have probably not been asked in a long time.
"The lesson for us today is that when you have chances, you have to take them against decent sides, Thailand certainly are a decent side and an international one at that, so it was a good education for us."