Young adults with psychiatric illnesses are more likely to use cannabis and experience problems from use. It is not known whether those with a lifetime psychiatric illness experience a prolonged cannabis withdrawal syndrome with abstinence. Participants were fifty young adults, aged 18–25, recruited from the Boston-area in 2015–2016, who used cannabis at least weekly, completed the Structured Clinical Interview for DSM-IV to identify Axis I psychiatric diagnoses (PD+ vs PD−), and attained cannabis abstinence with a four-week contingency management protocol. Withdrawal symptom severity was assessed at baseline and at four weekly abstinent visits using the Cannabis Withdrawal Scale. Cannabis dependence, age of initiation, and rate of abstinence were similar in PD+ and PD− groups. There was a diagnostic group by abstinent week interaction, suggesting a difference in time course for resolution of withdrawal symptoms by group, F(4,46)=3.8, p=0.009, controlling for sex, baseline depressive and anxiety symptoms, and frequency of cannabis use in the prior 90 days. In post hoc analyses, there was a difference in time-course of cannabis withdrawal. PD− had significantly reduced withdrawal symptom severity in abstinent week one [t(46)=−2.2, p=0.03], while PD+ did not report improved withdrawal symptoms until the second abstinent week [t(46) =−4.1, p=0.0002]. Cannabis withdrawal symptoms improved over four weeks in young people with and without a lifetime psychiatric diagnosis. However, those with a psychiatric illness reported one week delayed improvement in withdrawal symptom severity. Longer duration of cannabis withdrawal may be a risk factor for cannabis dependence and difficulty quitting.