Necroptosis is an inflammatory form of programmed cell death that has been widely implicated in human disease, including renal, pulmonary, gut, and skin inflammatory pathologies. The terminal effector of necroptosis is the pseudokinase MLKL (mixed lineage kinase domain-like). Upon phosphorylation by the kinase RIPK3, the cell killing activity of MLKL is activated, leading to the permeabilization of the plasma membrane to induce necroptotic cell death. However, the precise molecular mechanism underlying MLKL activation is only emerging. By combining cellular functional studies with structural insights through X-ray crystallography, electron microscopy, and computational modelling, we synthesized a grand unified model to the activation mechanism of MLKL. We identified four critical steps to the activation of MLKL by RIPK3: (1) a pre-necroptotic RIPK3:MLKL assembly under basal conditions; (2) a conformational change of MLKL upon phosphorylation, which leads to its disengagement from RIPK3; (3) the dimerization of phosphorylated pseudokinase domains; (4) this subsequently drives the formation of an elongated MLKL tetramer, thereby unleashing the cell-killing domains. Using mutational analyses we identified a variety of critical residues that mediate the MLKL:RIPK3 interaction pre- and post-activation and the oligomeric states of MLKL. We leveraged our new mechanistic insights to understand how the naturally occurring human MLKL polymorphism Ser132Pro, that is carried by 2-3% of humans around the globe, confers a gain in necroptotic function. We observed this MLKLS132P mutant exhibited gain-of-function in the presence of natural and pharmacological inhibition and this manifested in in vivo hematopoietic dysfunction. Taken together, our study unifies the structural and biochemical understanding of human MLKL activation. These findings will enable more nuanced investigations to dissect how mutations may alter MLKL regulation and thus, contribute to the progression of inflammatory pathologies.