During pre-implantation development, the mammalian embryo self-organizes into the blastocyst consisting of an epithelial layer encapsulating the inner-cell mass (ICM), which gives rise to all embryonic tissues 1. In mice, oriented cell division, apico-basal polarity and acto-myosin contractility are thought to contribute to the formation of the ICM 2– 5. However, how these processes work in concert remains unclear. Here, we show that asymmetric segregation of the apical domain generates blastomeres with different contractility, which triggers their sorting into inner and outer positions. 3D physical modeling of embryo morphogenesis reveals that cells internalize only when differences in surface contractility exceed a predictable threshold. We validate this prediction using biophysical measurements and successfully re-direct cell sorting within the developing blastocyst using maternal myosin ( Myh9) knockout chimeric embryos. Finally, we find that loss of contractility causes blastomeres to show ICM-like markers regardless of their position. In particular, contractility controls Yap sub-cellular localization 6, raising the possibility that mechanosensing occurs during blastocyst lineage specification. We conclude that contractility couples the positioning and fate specification of blastomeres. We propose that this ensures the robust self-organization of blastomeres into the blastocyst, which confers remarkable regulative capacities to mammalian embryos.