Most governments implement the latest information communication technology (ICT) to improve the online experience of their citizens and businesses. Governments put great effort into providing user-focused services that are usable, secure and accessible by portable and wireless devices (e.g. tablets, smart phones etc.). However, such devices bring with them specific problems of usability and security that affect how users interact with government digital services (GDS). This paper presents a systematic mapping study, investigating the existing problems of usability and security of GDS accessed through smart devices. It aims to uncover what evaluation methods have been used by researchers and investigate how the trade-off between usability and security is assessed in the context of GDS accessed through smart devices. The paper summarises the current knowledge available with regards to this trade-off over the last ten years. The results of the mapping study help identify several research gaps, leading to areas for new research in the domain of usability and security in the context of GDS.