Understanding the role of therapeutic process factors in treatment change may prove useful for discerning why some autistic youth benefit from psychosocial interventions that target emotional and psychological aspects of mental health, while others do not. The aim of the current study was to synthesize what is currently known about therapeutic process factors in mental health treatment of emotional and psychological challenges for autistic youth, regarding how process factors have been measured in past research, and the relation between process factors and treatment outcome. A systematic review of the literature was conducted to narratively synthesize all articles published up until June 2021. Methodological quality of included studies was appraised. Twenty-five studies met inclusion criteria. Process factors assessed across studies included relational factors; treatment expectations, readiness, and satisfaction; and treatment engagement from youth and their parents. Process-outcome associations were reported for a limited number of constructs. There is a limited, albeit growing, body of high-quality research evaluating the role of process factors in the treatment of mental health issues for autistic youth. Future research should continue to examine process factors in relation to treatment outcome, and validate measures to accurately capture process-related constructs in mental health treatment for this population. Greater understanding of therapy processes can lead to developing evidence-informed strategies that clinicians can implement to promote positive expectations, relationships, and engagement.