Myeloid cell leukemia-1 (MCL1) is among the most overexpressed proteins in tumors. MCL1 contributes to tumorigenesis by antagonizing apoptosis. However, apoptosis-unrelated functions are emerging. Screening an array of signaling switches identifies mTORC1 to be modulated by MCL1 but not by the anti-apoptotic Bcl-2 or Bcl-xL. mTORC1 is a central metabolic regulator. MCL1 impacts metabolism via modulating the expression of hexokinase 2 (HK2) in an mTORC1-dependent manner, which ultimately contributes to the tumor-promoting effects of MCL1. MCL1 inhibitors suppress mTORC1 in tumor cells but are associated with cardiotoxicity due to mTORC1 inhibition in the heart. Dietary leucine supplementation rescues mTORC1 signaling in the hearts of humanized Mcl-1 mice and greatly ameliorates the cardiotoxicity of MCL1 inhibitors. Taken together, here we describe tumor-promoting roles for MCL1 in regulating mTORC1 signaling and subsequently in bioenergetics, besides its role in antagonizing apoptosis, identifying MCL1 as a hinge of cell bioenergetics and survival.