In recent years, the growing popularity of ultramarathon events (endurance footraces longer than the 42.2km marathon) has attracted a range of participants from elite endurance athletes to recreational runners. Using a phenomenological approach, this study addresses a gap in the existing ultramarathon literature by highlighting experiences of first-time ultramarathon participants. This research builds on our understanding of ultrarunners’ psychology by exploring the development and application of mental skills used by these athletes while training for and competing in an ultramarathon. Eight individuals participated in this study, with data collection taking place at three time-points in their journey via a pre-race questionnaire, post-race journal, and follow-up one-on-one virtual interview. Qualitative data from interview transcripts and journals were analyzed together using reflexive thematic analysis, and results are presented in five major themes: (1) Preparation for the Race, (2) Physical Challenges, (3) Mental Challenges, (4) Relationships and Support, and (5) Personal Transformation. These results highlight a range of mental skills (e.g., goal-setting, imagery, self-talk, and attentional control) used by participants in their first ultramarathon. Barriers and facilitators for developing these skills are discussed (e.g., access to course-specific information). Results suggest that training for and completing in one’s first ultramarathon can be a transformative experience with impacts beyond the sport. The findings from this research have practical applications for athletes, coaches, mental performance consultants, and race directors. It also offers insight on the development of mental skills that has applications beyond the ultramarathon, for example when persevering through other challenging life events.