Thesis (Ph. D., Education) | This dissertation describes two studies based on data collected during a 3-month, season-long enjoyment-focused mental skills training (MST) intervention with a team of Division I female soccer players. Study 1 evaluates the effectiveness of the season-long intervention utilizing quantitative data to compare treatment and control groups. Study 2 describes how and why the intervention was implemented, utilizing quantitative and qualitative data from the treatment group to identify effect "process" variables. Repeated measures multivariate analysis of variance results from Study 1 revealed that the MST intervention significantly increased sport enjoyment, key mental training skills, and athletic performance both, in practice and competition, compared to the control group. Results from Study 2 demonstrated through case study, cross-case analyses and individual engagement scores how a MST program can enhance the quantity and quality of sport sources of enjoyment (SOE), the primary SOE present in collegiate soccer, autonomy-supportive SOE, the role of athletic engagement and how different components become more prominent at different points of the intervention, and sport enjoyment's impact on athletic performance. The discussion focuses on the role of the working implementation model in intervention effectiveness.