Research on the effectiveness of community-based mental health treatment for offenders with a serious mental illness (SMI) has lacked specifics on the type and intensity of services received. This study examined the detailed lifetime and past-year community mental health treatment use of 431 (282 men; 149 women) jail detainees with SMI. Whereas a majority of participants reported high lifetime rates of mental health treatment they believed effective, treatment in the year and month prior to arrest and detention was accessed by only a minority of the sample. Where gender differences were found, women were less likely to receive treatment than men and more likely to leave treatment against medical advice in the year preceding arrest. Both substance use disorders and post-traumatic stress disorder (PTSD) were significantly undertreated for both genders, and care was provided predominantly by psychiatrists suggesting an underuse of other mental health professionals. The implications for expanding treatment availability for offenders, particularly women, with an emphasis on broadening access to ancillary but critical services such as literacy training, housing and employment services, and case management provided by mental health professionals in conjunction with services provided by psychiatrists are discussed.