Though a growing field, adolescent and young adult (AYA) cancer survivorship research has several knowledge gaps. For instance, many scientists report challenges recruiting AYAs cancer survivors to intervention research, and little is known regarding what motivates AYAs who do participate in studies. Using data from the THRIVE intervention study, this study analyzed the associations between AYA cancer survivors’ sociodemographic characteristics and motivations for joining a mobile physical activity (PA) intervention as well as the associations between these motivations and PA attainment in the study. Results of a series of chi-square analyses revealed associations between education and PA-related motivations, income and PA-related motivations, stage at diagnosis and research interest motivation, insurance and research interest motivation, and income and research interest motivation. Further, results of ANOVA analyses do not support an association between motivations for joining a mobile PA intervention and PA outcomes in the study. Consistent with prior research, over 60% of participants in this study endorsed PA-related, weight-related, and research interest motivations for joining the study. Study findings provide insight into ways to improve recruitment strategies to engage AYA cancer survivors in future PA interventions.