Owen Cleary, who taught at the city's Monnow junior school, appeared before a General Teaching Council for Wales conduct committee last month.
He was found guilty of unacceptable professional conduct for not reporting a 2002 conviction for threatening behaviour and a 2003 caution for another public order offence. He did disclose a 1999 conditional discharge for criminal damage.
A Newport council spokesman said Mr Cleary had successfully applied for a two-year fixed term contract at the school in 2003, but missed out on a permanent job in 2004. When his full criminal record came to light, he was dismissed and left the school earlier this year.