Police launched a manhunt and Furlong, as well as Canadian woman Brittney Schneider, 23, who was with him at the time and also sprayed "B" on the wall, were arrested Thursday afternoon.
The words "Scouser Lee" were painted on the wall, seemingly a reference to the city of Liverpool.
The 23-year-old holidaymaker, named Lee Furlong, was travelling in Chiang Mai and mis-spelt "Scouse" on the Thai "Tha Phae" red-bricked gate.
Pol Col. Teerasak Sriprasert, chief of the Chiang Mai police, said the tourists are in custody and have been charged with vandalizing registered ancient artifacts, which carries a penalty of up to 10 years in prison and a million baht ($30,650) fine.
He said the pair have admitted spraying the message on the wall.More news: Fall book sale at Pawtucket Library
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Schneider said her daughter told her she was drunk and she was following along. She said that the "routine" action, which looks like a march of shame, would have been the case if it was domestic or foreign-born suspects.
Schneider said she asked her daughter why she did what she did. "She is liked by many, she's got friends everywhere in the country. But I said stay strong and breathe and go through it". "She's never been in trouble in her life".
Canadian consular officials have arrived in the city and are speaking with the suspects, according to Global Affairs Canada.
Schneider's mom said they have been working with the consulate.
Bail money has been sent to Thailand so Brittney can be released on Monday, she said.