This study examined the association between pre- or early-adolescent onset of gambling and severity of gambling and psychosocial problems in treatment-seeking adult pathological gamblers. A total of 236 pathological gamblers entering outpatient treatment completed the South Oaks Gambling Screen (SOGS) and the Addiction Severity Index (ASI). Using a quartile split procedure, gamblers who began gambling during their pre- or early-adolescent years (mean age of 10.5 years; 1st quartile) were compared to gamblers who began gambling later in life (mean age of 23.0 years; 2nd to 4th quartiles). Compared to later onset gamblers, pre/early adolescent onset gamblers reported increased severity of psychiatric, family/social, and substance abuse problems on the ASI. They were more likely to report cognitive problems (trouble understanding, concentrating, or remembering), suicidal ideation, and a history of inpatient psychiatric treatment, and were less likely to be satisfied with their current living situation. Pre/early adolescent onset gamblers also reported earlier age of initiation of drinking, and were more likely to have received treatment for an alcohol use disorder, and to have used cannabis and cocaine in their lifetimes. Taken together, these data suggest that pre/early adolescent-onset of gambling may be a risk factor for later-life psychiatric, family/social, and substance abuse problems in treatment-seeking pathological gamblers.