Parnham has practiced law for thirty-seven years in Houston, Texas, specializing in criminal defense. Viewing law as a practical way to serve his fellow citizens, he has dedicated his life to the defense of the underdog. He has become an expert on the defense of individuals with mental illness and a passionate advocate for legal reform of their treatment in the criminal justice system. Parnham is called upon frequently by local and national news media to render an opinion on insanity plea cases and general defense issues in high profile cases, such as Lee Boyd Malvo (one of the accused D.C. snipers), Robert Durst, Kobe Bryant, and Scott Peterson.