Informed by athletes' psychological needs, the current qualitative study developed, implemented, and evaluated a 15-month mental skills training (MST) program for elite youth athletes. The MST was divided into three phases that included a nine-month pre-intervention, a two-month intervention, and a four-month evaluation phase. The intervention consisted of three interactive workshops which were delivered to 11 competitive British youth tennis players (aged 8 to 15 years) and their coach (age = 34). The intervention was informed by data that was collected throughout a nine-month pre-intervention phase including longitudinal observations, field notes, and semi-structured interviews. The intervention was evaluated over a four-month period through observations, field notes, athlete-workshop data, and a semi-structured interview with the coach. Results reinforced the value of the longitudinal pre-intervention phase by highlighting that the establishment of rapport between the researcher and athletes enhanced the meaningfulness and content of the MST program. Additionally, an increase in athletes' use and regulation of PSCs was identified as a result of athletes' improved understanding of psychological skills (i.e., self-talk, imagery, performance routines) and characteristics (i.e., focus, emotional control) (PSCs). Finally, the MST program fostered a shared subject specific language between athletes and their coach, enhancing the openness and frequency with which PSCs were talked about. Practical guidelines for future sport psychology interventions with youth athletes and their coaches are provided. Lay Summary: A 15-month mental skills training program was conducted with youth tennis players to enhance their ability to regulate their focus and emotional control. Practical guidelines for future interventions with youth athletes and their coaches are provided.