Millon was born in 1928, the only child of immigrant Jewish parents from Lithuania and Poland. His 19th-century ancestors came from the town of Valozhyn, then a part of the Russian Empire. Receiving degrees from both American and European universities, he was a member of the board of trustees of Allentown State Hospital, a large Pennsylvania psychiatric hospital for 15 years. Shortly thereafter he became the founding editor of the Journal of Personality Disorders and the inaugural president of the International Society for the Study of Personality Disorders. He is Professor Emeritus at Harvard Medical School and the University of Miami.

In 2008, Millon was awarded the Gold Medal Award For Life Achievement in the Application of Psychology by the American Psychological Association.

The American Psychological Foundation presents an award named after Millon, known as the "Theodore Millon Award in Personality Psychology," to honor outstanding psychologists engaged in "advancing the science of personality psychology including the areas of personology, personality theory, personality disorders, and personality measurement."

Millon devised a set of widely acknowledged subtypes for each of the DSM personality disorders:

• Sadistic personality disorder subtypes
• Self-defeating personality disorder subtypes
• Schizotypal personality disorder subtypes
• Schizoid personality disorder subtypes
• Paranoid personality disorder subtypes
• Antisocial personality disorder subtypes
• Borderline personality disorder subtypes
• Histrionic personality disorder subtypes
• Narcissistic personality disorder subtypes
• Dependent personality disorder subtypes
• Obsessive-compulsive personality disorder subtypes
• Avoidant personality disorder subtypes
• Passive-aggressive personality disorder subtypes
• Depressive personality disorder subtypes
• Exuberant/Hypomanic personality disorder subtypes