Eating disorders are widespread illnesses with significant global impact. There is growing concern about how young people overuse online resources leading to mental health sequelae. We gathered data from 639 individuals from a population cohort. Participants were all young adults at the point of contact and were grouped as having probable eating disorder with excessive exercise (n = 37) or controls (n = 602). We measured obsessionality, compulsivity, impulsivity, and problematic internet use. Group differences in these domains were evaluated; and structural equation modelling (SEM) was used to assess structural relationships between variables. Cases had higher scores of obsessional thoughts of threat (Cohen's d = 0.94, p < 0.001), intolerance towards uncertainty (Cohen's d = 0.72; p < 0.001), thoughts of importance and control (Cohen's d = 0.65, p < 0.01), compulsivity (Cohen's d = 0.72; p < 0.001), negative urgency (Cohen's d = 0.75, p < 0.001), and higher problematic usage of the internet (Cohen's d = 0.73; p-corrected <0.001). Our SEM showed significant partial mediation of problematic internet use on both the effect of obsessionality latent factor on cases (z-value = 2.52, p < 0.05), as well as of sensation seeking latent factor on cases (z-value = 2.09, p < 0.05). Youth with eating disorder and heightened exercise levels have increased obsessive thoughts of threat, compulsivity traits and sensation seeking impulsivity. The association between obsessive thoughts and eating disorders, as well as sensation seeking and eating disorders were partially mediated by problematic internet use. Problematic internet use may be playing a role in the development or maintenance of eating disorder symptoms in the background of obsessional thoughts and sensation seeking impulsive traits.