Mast cells originate from the bone marrow and develop into c-kit(+) FcɛRI(+) cells. Both mast cell progenitors (MCp) and mature mast cells express these cell surface markers, and ways validated to distinguish between the two maturation forms with flow cytometry have been lacking. Here, we show that primary peritoneal MCp from naïve mice expressed high levels of integrin β7 and had a low side scatter (SSC) light profile; whereas mature mast cells expressed lower levels of integrin β7 and had a high SSC light profile. The maturation statuses of the cells were confirmed using three main strategies: (1) MCp, but not mature mast cells, were shown to be depleted by sublethal whole-body γ-irradiation. (2) The MCp were small and immature in terms of granule formation, whereas the mature mast cells were larger and had fully developed metachromatic granules. (3) The MCp had fewer transcripts of mast cell-specific proteases and the enzyme responsible for sulfation of heparin than mature mast cells. Moreover, isolated peritoneal MCp gave rise to mast cells when cultured in vitro. To summarize, we have defined MCp and mature mast cells in naïve mice by flow cytometry. Using this strategy, mast cell maturation can be studied in vivo.