Population ageing has intensified the need for transport systems that support independent and inclusive mobility among older adults. In urban environments, mobility challenges often arise from declining health, driving cessation, and structural barriers within public transport systems. While on-demand mobility (ODM) services have emerged as flexible alternatives, empirical evidence on their adoption among older adults remains limited, particularly in developing contexts. This study examines the determinants of ODM usage among older adults in the Klang Valley, Malaysia. Using cross-sectional survey data from 497 respondents aged 60 years and above, binary logistic regression was employed to estimate the probability of ODM adoption. The findings reveal that perceived public transport barriers are the most significant determinant of ODM use. Older adults facing difficulties related to accessibility, waiting time, and boarding conditions are substantially more likely to rely on ODM services. Driving ability also plays a significant role, with active drivers being less likely to adopt ODM, indicating a substitution effect following driving reduction or cessation. In contrast, socio-demographic and health-related factors do not show independent significant effects. Overall, the results suggest that ODM functions as a compensatory mobility mechanism in response to structural deficiencies in public transport systems, highlighting the importance of age-friendly and integrated transport strategies in rapidly ageing urban regions.