Born on November 10, 1920, in New Orleans, Louisiana, Mentz received a Bachelor of Arts degree from Tulane University in 1941 and a Bachelor of Laws from the Paul M. Hebert Law Center at Louisiana State University in 1943.

He served in private practice in Hammond, Louisiana from 1946 to 1982.

Concurrent with his private practice, he served as a staff attorney for the Shell Oil Company from 1947 to 1948, as an assistant to the executive counsel to the Governor of Louisiana in 1948 and as the city attorney of Hammond from 1954 to 1961.

[1] Mentz was nominated by President Ronald Reagan on June 2, 1982, to a seat on the United States District Court for the Eastern District of Louisiana vacated by Judge Lansing Leroy Mitchell.

His service terminated on December 31, 2001, due to retirement.