He served in the United States Marine Corps from 1971 to 1973 and fought in two tours in the Vietnam War, then graduated with a Bachelor of Business Administration from the University of Maryland in January 1979, and joined the FBI in September of the same year.

[2] In late 1985 and early 1986 two armed men had robbed a series of banks, and killed a number of people near Florida.

After the two had gotten into a police vehicle, Mireles stood up, went towards them and shot the two of them dead at short range with his personal .357 Magnum S&W Model 686 revolver.

His left arm remained impaired, but he returned to the service after a year of recovery and became an instructor at the FBI Academy in Quantico, Virginia.

[citation needed] Along with more than half the agency's Hispanic staff, he was involved in a 1988 discrimination suit against the FBI.