Background Serious games have the potential to transform the field of cognitive assessment. The use of serious game–based cognitive assessments in prison environments is particularly exciting. This is because interventions are urgently needed to address the rapid increase in the number of currently incarcerated older adults globally and because of the heightened risks of dementia and cognitive decline present in this population. Game-based assessments are assumed to be fun, engaging, and suitable alternatives to traditional cognitive testing, but these assumptions remain mostly untested in older adults. This is especially true for older adults in prison, whose preferences and needs are seldom heard and may deviate from those previously captured in studies on cognition and serious games. Objective This study aimed to understand the design preferences of older adults in prison for a game-based cognitive assessment. Methods This study used reflexive thematic analysis, underpinned by critical realism, and applied the technique of abduction. Overall, 4 focus groups with a total of 20 participants were conducted with older adults (aged ≥50 years; aged ≥45 years for Aboriginal and Torres Strait Islander people) across 3 distinct prison environments in Australia. Results Self-determination theory was used as a theoretical foundation to interpret the results. Overall, 3 themes were generated: Goldilocks—getting gameplay difficulty just right through optimal challenge (the first theme emphasizes the participants’ collective desire for an individualized optimal level of difficulty in serious gameplay), Avoiding Childish Graphics—gimmicky gameplay can be condescending (the second theme raises the importance of avoiding immature and childlike gameplay features, as some older end users in prison felt that these can be condescending), and A Balanced Diet—meaningful choice and variety keeps game-based assessments fun (the third theme highlights the strong user preference for meaningful choice and variety in any serious game–based cognitive assessment to maximize in-game autonomy). Conclusions The collection of these themes provides novel insights into key game design preferences of marginalized older adults.