He was long associated with the Payne Whitney Psychiatric Clinic in New York, serving as the head of its adolescent program in the 1960s and 1970s, where he developed his theory of borderline personality disorder.
In so doing, he helped widen the lens through which disturbance in mental health is viewed, seeing beyond the classical Freudian approach, which is limited to neurotic symptomatology.
In work he began in the mid-20th century, Masterson came to believe that the origin and treatment of personality disorders starts with understanding the formation of self and relationship that begins in the first three years of life—the so-called preOedipal period.
The psychoanalysts Otto F. Kernberg, Ronald Fairbairn, D.W. Winnicott and Heinz Kohut also played seminal roles in developing the concept of personality disorder.
Masterson founded his psychotherapeutic approach while facing the challenge of finding an effective way of working with an inpatient unit of acting-out adolescents.
Masterson theorized that the dynamic structure of the borderline reflected a developmental arrest in the stage of childhood development described by Margaret Mahler as the "rapprochement" sub-phase: a time when the two-year-old is caught between unresolved urges toward dependence and independence.
The closet narcissist is more likely to be described as having a deflated, inadequate self perception and greater awareness of emptiness within, and seeks to mend this with an unquestioning dedication to an idealized other.