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Full implementation of phylogenetic classifications remains pending, particularly to delimit infrageneric divisions of large genera. Mimosa, one of the largest genera of Fabaceae, includes five sections and 41 series, most of which are not monophyletic. Here we investigated the phylogenetic relationship among species of Mimosa series Pachycarpae and Setosae, two diverse series from the Brazilian Cerrado (savanna) Domain. We analysed morphological and molecular data for a wide taxonomic sample in a total-evidence approach. Our results show the non-monophyly of these series is more problematic than previously realized and extends to M. series Piresianae. Nonetheless, most taxa of M. series Pachycarpae and Setosae fall in a clade, which has an enlarged underground organ as one of its synapomorphies and an important functional trait underlying adaptation to fire in the Cerrado Domain. On the basis of these results, and after transferring some species to M. series Piresianae, we synonymize M. series Setosae under M. series Pachycarpae. These updates are a first step towards aligning the infrageneric classification of Mimosa with the tenets of phylogenetic systematics.