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.
On 6 January 1986 he was consecrated bishop by Pope John Paul II in Rome.
In April 2000 he met with the Superior General of the Society, Bishop Bernard Fellay.