This study explored the relationship between Fear of Missing Out (FoMO) and irrational procrastination in a mobile social media environment and its underlying mechanism: the mediating role of cognitive failure. The study was conducted with 817 college students using the FoMO Scale, Irrational Procrastination Scale, Cognitive Failures Questionnaire, and Self-Control Scale. The results showed that (a) FoMO positively predicted irrational procrastination in the mobile social media environment; (b) cognitive failure had a complete mediating effect on the relationship between FoMO and irrational procrastination; and (c) self-control had a moderating effect on the relationship between FoMO and cognitive failure.