Mental toughness (MT) is a key psychological variable related to achievement in performance domains and perseverance in challenging circumstances. We sought to understand the lived-experiences of mentally tough high-altitude mountaineers, focusing primarily upon decisions to persevere or abort summit attempts. Phenomenological interviews were conducted with 14 mountaineers including guides, expedition leaders, and doctors (Mage = 44 years). A content analysis was employed to identify key themes in the data. Participants emphasised the importance of MT in extreme environments and described rational, flexible, and vigilant decision-making. Turning around without summiting was the toughest decision reported, with recognition of the thin line between persevering and over-stretching. In contrast to much MT literature, mountaineers accepted limits, demonstrated restraint, and sacrificed personal goals to aid others. Costly perseverance was also reported with some mountaineers described as "too tough": over-competitive, goal-obsessed, and biased decision-makers. These findings revealed both benefits and dangers of MT in mountaineering.