Background: Infant regulatory problems (RPs), i.e., problems with crying, feeding, and/or sleeping, are associated with behavioral and emotional problems in childhood. It is unclear, however, whether these behavioral and emotional problems persist into adulthood. The default mode network (DMN) and salience network (SN) support both interoceptive regulation and social and emotional abilities. We thus hypothesized that adults who had experienced RPs in infancy have more behavioral and emotional problems, which are mediated by DMN and/or SN alterations. Methods: Within the scope of the Bavarian Longitudinal Study, adults (mean age 28 years; 50% female subjects) with (n = 79) and without (n = 254) a history of multiple and/or persistent infant RPs were assessed by the Young Adult Self Report to measure behavioral and emotional problems, and—in a subsample (n = 49 with and n = 71 without a history of infant RPs)—by resting-state functional magnetic resonance imaging to measure DMN/SN integrity via intrinsic functional connectivity (iFC). Results: Compared with adults with no history of infant RPs, adults who had experienced infant RPs had more total problems (p =.002), more internalizing problems (p =.005), and more avoidant personality traits (p <.001). They showed decreased iFC of the DMN and SN. DMN iFC decreases were strongest in adults with multiple and persistent RPs, and they were linked with avoidant personality traits (r = −.42, p =.006). Remarkably, DMN iFC decrements fully mediated the association between infant RPs and adult avoidant personality traits. Conclusions: Adults who had experienced infant RPs have more avoidant personality traits that are mediated by the DMN. Persistent and/or multiple infant RPs and the DMN may be targets to attenuate behavioral and emotional problems.