 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.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.He was confirmed by the United States Senate on June 24, 1982, and received commission on June 25, 1982.