Although the number of older adults who are arrested and subject to incarceration in jail is rising dramatically, little is known about their emergency department (ED) use or the factors associated with that use. This lack of knowledge impairs the ability to design evidence-based approaches to care that would meet the needs of this population. This 6-month longitudinal study aimed to determine the frequency of 6-month ED use among 101 adults aged 55 or older enrolled while in jail and to identify factors associated with that use. The primary outcome was self-reported emergency department use within 6 months from baseline. Additional measures included baseline socio-demographics, physical and mental health conditions, geriatric factors (e.g., recent falls, incontinence, functional impairment, concern about post-release safety), symptoms (pain and other symptoms), and behavioral and social health risk factors (e.g., substance use disorders, recent homelessness). Chi-square tests were used to identify baseline factors associated with ED use over 6 months. Participants (average age 60) reported high rates of multimorbidity (61%), functional impairment (57%), pain (52%), serious mental illness (44%), recent homelessness (54%), and/or substance use disorders (69%). At 6 months, 46% had visited the ED at least once; 21% visited multiple times. Factors associated with ED use included multimorbidity (p = 0.01), functional impairment (p = 0.02), hepatitis C infection (p = 0.01), a recent fall (p = 0.03), pain (p < 0.001), loneliness (p = 0.04), and safety concerns (p = 0.01). In this population of older adults in a county jail, geriatric conditions and distressing symptoms were common and associated with 6-month community ED use. Jail is an important setting to develop geriatric care paradigms aimed at addressing comorbid medical, functional, and behavioral health needs and symptomatology in an effort to improve care and decrease ED use in the growing population of criminal justice-involved older adults.