When partners coordinate their movement in time and space to reach a goal, they perform joint action, an important part of every interaction. Joint action involves motor abilities and socio-cognitive skills like theory of mind. Autistic children's lower joint motor coordination (joint action) abilities as well as their motor functioning and theory of mind difficulties may interfere with efficient peer interaction. However, the shared contribution of motor and theory of mind to partners' joint action was not yet explored. This study investigated those contributors (motor and theory of mind) along with group and age differences in 84 autistic children ages 6-16 years and 64 non-autistic children matched by age, sex, and IQ across three age-groups: early-childhood, preadolescence, and adolescence. Basic and advanced theory of mind skills and most motor tasks were higher among adolescents versus early-childhood. However, the autistic group consistently underperformed the non-autistic group in basic and advanced theory of mind levels and in all gross- and fine-motor tasks across all age-groups, revealing unique motor development characteristics in autism. A significant joint full mediation effect emerged for motor and theory of mind skills on joint action performance in both study groups. Understanding that motor and theory of mind skills together underlie joint action opens up a new channel of intervention to facilitate peer interaction.