Background: The growth among older prisoner populations, including in Australia, necessitates an understanding of this group in order to generate effective management strategies. One particular concern is the mental well-being of older prisoners. This study aimed to determine the level of psychological distress among sentenced prisoners aged 50 years and older, to compare this level to that seen among younger prisoners and older people in the community, and to investigate which mental health history, cognitive functioning, socio-demographic, and criminal justice characteristics were associated with psychological distress. Methods: A cross-sectional survey of 173 older (M = 63 years) and 60 younger prisoners (M = 34 years) in two Australian jurisdictions was conducted. The Kessler Psychological Distress (K10) scale was administered with prisoners and additional data were collected from interviews and participant health and corrections files. K10 scores were compared to community norms using data from the Australian Health Survey. Results: Average K10 scores of the older prisoners were significantly lower than the younger prisoners (p = 0.04), though the effect size was small (r = 0.1). Significantly, higher distress levels were observed in comparison to the general population (p <0.001), with older prisoners being three times more likely to display very high levels of distress (12.3 vs. 3.7 ). Higher psychological distress scores among older prisoners were significantly associated with female gender (p = 0.002) and a history of mental health issues (p = 0.002). Conclusions: While the levels of distress seen among older prisoners were significantly lower than that of younger prisoners, their higher levels of distress in comparison to community norms demonstrate a need for correctional services to be attuned to the mental health of the expanding older prisoner population.