We used longitudinal data from 205 members of 53 student teams who competed in a complex business simulation over 10 weeks to test: (1) whether shared leadership and performance were related reciprocally over time; (2) the relative magnitude of those relationships; and (3) whether a shared leadership intervention changes those relationships. We also considered the influence of team members’ mean level personality to account for compositional effects. As anticipated, shared leadership and performance were related positively and reciprocally over time. However, the shared leadership performance relationship was significantly higher than the reverse. Moreover, the shared leadership performance relationship grew stronger whereas the performance shared leadership relationship remained fairly consistent over time. As expected, the intervention positively impacted shared leadership and the shared leadership performance relationship. In addition, we find team mean levels of agreeableness play a role in intervention strength.