After ordination he worked in parish, school, and seminary posts in the diocese of Ratchaburi.
In 1976/77 he studied at The Catholic University of America in Washington, D.C., from which he obtained a master's degree in philosophy.
On 25 November 1985 Manat was appointed bishop of Ratchaburi in succession to the late Joseph Ek Thabping.
[2] In May 1993 during a visit to Manila in the Philippines, Manat came into contact with the Society of Saint Pius X.
In April 2000 he met with the Superior General of the Society, Bishop Bernard Fellay.