Minimal research has examined psychological processes underpinning ultra-marathon runners' performance. This study examined the relationships between mental toughness and self-efficacy with performance in an elite sample of ultra-marathon runners competing in the 2019 Hawaiian Ultra Running Team's Trail 100-mile endurance run (HURT100). The Mental Toughness Questionnaire (SMTQ) and the Endurance Sport Self-Efficacy Scale (ESSES) were completed by 56 elite ultra-marathon runners in the HURT100 (38 males, 18 females; Mage = 38.86 years, SDage = 9.23). Findings revealed mental toughness and self-efficacy are highly related constructs (r(54) = 0.72, p < 0.001). Mental toughness and self-efficacy did not significantly relate to ultra-marathon performance (mental toughness and self-efficacy with Ultra-Trail World Tour (UTWT) rank F(2, 53) = 0.738, p = 0.483; mental toughness and self-efficacy with likelihood would finish the HURT100 χ2 = 0.56, p = 0.756; mental toughness and self-efficacy with HURT100 placing and time F(2, 53) = 1.738, p = 0.186 and F(2, 30) = 2.046, p = 0.147, respectively). However, participants had significantly and meaningfully higher mental toughness (M = 45.42, SD = 4.26, medium and large effect sizes) than athletes from other sports previously published. Our interpretation is that these results taken in conjunction, suggest a threshold of mental toughness that performers require to be of the standard needed to be able to prepare for and compete in elite ultra-marathon events such as the HURT100; once this mental toughness threshold is met, other factors are likely to be more influential in determining elite level ultra-marathon performance.