The definition and classification of mental disorders are key issues for researchers as well as service providers and those who may be diagnosed. For a mental state to be classified as a disorder, it generally needs to cause dysfunction.[15] Most international clinical documents use the term mental "disorder", while "illness" is also common. It has been noted that using the term "mental" (i.e., of the mind) is not necessarily meant to imply separateness from the brain or body.
According to the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), published in 1994, a mental disorder is a psychological syndrome or pattern that is associated with distress (e.g., via a painful symptom), disability (impairment in one or more important areas of functioning), increased risk of death, or causes a significant loss of autonomy.
In 2013, the American Psychiatric Association (APA) redefined mental disorders in the DSM-5 as "a syndrome characterized by clinically significant disturbance in an individual's cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental processes underlying mental functioning."
I dunno, sounds to me like autism fits fine with "mental illness", possibly depending on the severity/placement on the spectrum. Note that mental illness isn't something easily defined. I just pulled the quotes above from Wikipedia.