The ant-like stone beetle supertribe Mastigitae (Staphylinidae: Scydmaeninae) comprises nine extant and five extinct genera that exhibit unusual morphological specializations and ecological adaptions. Recent discoveries of mastigite fossils from Upper Cretaceous-middle Eocene deposits have significantly improved our knowledge of the evolutionary history of this group, yet a direct fossil record for two modern tribes of unclear affinities, i.e. Leptomastacini and Papusini, is lacking. Herein we describe a new genus and species, dagger Archemastax divida gen. et sp. nov., based on two well-preserved individuals entombed in mid-Cretaceous Burmese amber (c. 99 Ma), representing the earliest known representative of Leptomastacini. dagger Archemastax possesses both plesiomorphic and derived characters, with a few important traits shared with extant members of Papusini and Clidicini. A data set of 70 morphological characters scored for 29 species of Mastigitae was analysed using maximum parsimony and Bayesian inference; both analyses recovered a sister relationship between dagger Archemastax and extant leptomastacines. However, in the former analysis Leptomastacini was recovered as sister to the remaining Mastigitae, while the latter indicated an origin of Leptomastacini from within Clidicini. Although a close affinity between dagger Archemastax and Papusini was not supported by our analyses, similarities in some shared characters of these two groups are discussed.