Researchers have examined how individuals' preferred ways of comparing achievements differ according to their mindset. However, studies investigating the concomitant impact of such differences on individuals' happiness are relatively scant. Using a variety of scenarios, the present study examined how fixed-mindset and growth-mindset individuals' different achievement-comparison styles (more than others, less than others, better than before, and worse than before) affected their subjective happiness. A total of 880 participants were recruited. Fixed-mindset individuals felt happy when they felt they achieved more than others and unhappy when they felt the opposite, but were not influenced significantly when their achievement was better or worse than before. Conversely, growth-mindset individuals felt happy when their achievement was better than before but unhappy when it was the opposite, without being influenced significantly by achieving more or less than others. This study examined mindset, achievement comparison, and subjective happiness comprehensively, which, to date, have only been examined independently.