Introduction: Methadone maintenance therapy (MMT) requires a client to take a daily dose of methadone to reduce cravings for opioid and suppress withdrawal syndrome. Non-retention in the MMT will seriously expose more risk rather than the benefits of the program. Hence, determining the factors associated with non-retention to the MMT is essential. Thus, the present study was conducted to determine the proportion of non-retention and its associated factors among clients receiving MMT in Kelantan, Malaysia. Methods: By using a retrospective record review analysis, the required information of the clients was obtained from fourteen MMT clinics in Kelantan, Malaysia. A total of 155 clients were selected in this study using systematic random sampling. Multiple logistic regression analysis was performed to determine the factors associated with non-retention among clients receiving MMT. Results: It was found that the proportion of non-retention to the MMT in Kelantan, Malaysia was 21.9%. Clients with advanced age, taking daily direct observed therapy, and no underlying HIV were the predictors of non-retention among clients receiving MMT in Kelantan, Malaysia. Conclusion: The findings highlight a relatively high proportion of non-retention among the MMT clients. There is a need for an integrated, culturally relevant approach towards tackling the factors associated with non-retention in the future to keep this program sustainable and effective.