 He began his clinical practice in 1991 at Columbia Presbyterian Medical Center in New York City, concentrating on heart transplantation and cardiovascular research.In 1994, Mills was named Chief of Pediatric Cardiac Anesthesiology at the University of Miami, where he was actively involved in both the recovery community and in the gay community as an advocate for people living with HIV.In 1999, he moved to Los Angeles and opened a general medical practice specializing in HIV care.