The present study examined the effects of a mental skills training package on competitive gymnasium triathlon performance and evaluated the utilization and impacts of the mental skills during performance. Four participants competed against each other on ten occasions in a single-subject multiple baseline across individuals design, which was used to evaluate an intervention package including goal setting, relaxation, imagery, and self-talk. The results indicated the mental skills package to be effective in enhancing all participants’ competitive triathlon performance and usage of mental skills from baseline to intervention phases. Qualitative data revealed that each of the mental skills were employed both prior to and during each triathlon and had varying impacts depending on when they were utilized. Issues regarding mental skill effectiveness and usage within competitive endurance performance are discussed.