To elucidate the signalling network responsible for mediating the effects of FGF21, we quantified the dynamic phosphoproteome of adipocytes following acute exposure to this hormone. A major FGF21-regulated signalling node was mTORC1/S6K. In contrast to insulin, we find that FGF21 activates mTORC1 via MAPK signalling rather than through the canonical PI3K/AKT pathway. This study provides a systems view of FGF21 signalling and reveals a new role for mTORC1 in maintaining nutrient homeostasis.