Cytotoxic T lymphocytes (CTLs) lyse target cells by delivering lytic granules that contain the pore former perforin to the cytotoxic immunological synapse. Here, we establish that opposing cytoskeletal forces drive lytic granule polarization and simultaneously shape T cell synapse topography to enhance target perforation. At the cell rear, actomyosin contractility drives the anterograde movement of lytic granules toward the nucleus. At the synapse, dynein-derived forces induce negatively curved membrane pockets to which granules are transported around the nucleus. These highly concave degranulation pockets are located directly opposite positively curved bulges on the target cell membrane. We identify a curvature bias in the action of perforin, which preferentially perforates positively curved tumor cell membrane. Together, these findings demonstrate murine and human T cell-mediated cytotoxicity to be a highly tuned mechano-biochemical system, in which the forces that polarize lytic granules locally bend the synaptic membrane to favor the unidirectional perforation of the target cell. [Display omitted] • T cells contract their tail to propel lytic granules anteriorly toward the nucleus • The forces that polarize lytic granules also shape T cell synapse membrane curvature • T cell lytic granules are transported to negatively curved degranulation pockets • Perforin preferentially perforates positively curved plasma membrane Govendir and Kempe et al. employ biophysical methods and quantitative imaging to study the mechanics of individual T cells killing tumor cells. The authors show that the cellular forces that polarize lytic granules also induce highly curved membrane pockets at the synapse, where perforin preferentially perforates positively curved target membrane. [ABSTRACT FROM AUTHOR]