Is paranoid personality disorder a true mental illness?

Yes, it is: Yes, paranoid personality disorder is a real mental illness. In it, there's a fixed, pervasive, and relatively inflexible pattern of distrust and suspiciousness such that others' motives are interpreted as malevolent. It generally begins in teen years or early adulthood, and differs markedly from one's culture. It causes distress in important areas of functioning, like relationships or work.