He was in private practice in Miami, Florida from 1933 to 1965 and served in the United States Naval Reserve as a lieutenant commander from 1942 to 1945, during World War II.
[1] President Lyndon B. Johnson nominated Mehrtens to the United States District Court for the Southern District of Florida on August 20, 1965, to a seat vacated by Judge Emmett Clay Choate.
He was confirmed by the United States Senate on August 31, 1965, and received his commission the next day.
[1] Judge Mehrtens presided over a case where the State of Florida claimed ownership over the wreck of the Nuestra Señora de Atocha, a Spanish Galleon carrying over $400 Million worth of treasure which had sunk in a hurricane in the 1700s.
On August 21, 1978, Judge Mehrtens, upon remand from the United States Court of Appeals for the Fifth Circuit, issued an opinion chastising the State of Florida's Division of Archives for using coercive backroom tactics to try and lay claim to treasure it knew it didn't belong to it.