The need to expand knowledge of family-based intervention for people with ASD has been observed. However, there is a lack of study in Chinese context such as Hong Kong exploring the use of multifamily therapy (MFT) for adults with ASD. The primary aim of the present study is to assess the feasibility of a process-oriented MFT in promoting family relationships for young adults with high functioning-ASD (HF-ASD) and their parents. We also sought to explore the therapeutic process factors of MFT that were deemed helpful by the family participants in our study. Two types of MFT—one online, the other face-to-face—were conducted from March to August 2021 for families with adults with HF-ASD in Hong Kong. In total, 13 families participated in our multi-method study. Among the results, cross-family therapeutic alliance and treatment engagement were positively associated with enhanced family relationships among participants. Significant differences emerged in changes in the cross-family therapeutic alliance and family relationships between the online and face-to-face MFTs. Two themes regarding the helpful aspects of MFT emerged: first, having a cross-family interactional context and, second, the authentic engagement of people with HF-ASD. Overall, our results imply that MFT, in either online or face-to-face mode, could be a feasible family group psychotherapy for adults with HF-ASD and their parents.