Yes, after his reign: No, but he initially aspired to be one
Answer
Mongkut was a monk before becoming king. He joined the Buddhist priesthood at the age of 20 and spent 27 years as a monk. During his time in the priesthood, he studied Buddhist philosophy and meditation. He also became fluent in several languages, including English, French, and Pali.
The Enigmatic Reign of Mongkut: A Captivating English Quiz on the King of Siam