This study aims to examine the mechanism of how supportive interaction and facilitator input variety in mutual aid groups impact treatment adherence of young people with chronic health conditions, with consideration of time effects, which have been rarely studied in the existing literature. A stratified random sample of 391 individuals aged 12–45 years with chronic health conditions were recruited from mutual aid groups in Hong Kong and completed both the baseline and 12-month follow-up surveys. The results of the structural equation modeling indicated that supportive interaction and facilitator input variety positively predicted treatment adherence in a delayed condition, whereas members’ treatment adherence in the baseline survey had reversed effects on members’ supportive interaction in the follow-up survey. The findings of this study shed light on the dynamic mechanism of the mutual aid groups and provide important implications to promote better rehabilitation outcomes of young people with chronic health conditions.