Background Severe cardiac cachexia or malnutrition are commonly considered relative contraindications to left ventricular assist device (LVAD) implantation, but post‐LVAD prognosis for patients with cachexia is uncertain. Methods and Results Intermacs (Interagency Registry for Mechanically Assisted Circulatory Support) 2006 to 2017 was queried for the preimplantation variable cachexia/malnutrition. Cox proportional hazards modeling examined the relationship between cachexia and LVAD outcomes. Of 20 332 primary LVAD recipients with available data, 516 (2.54%) were reported to have baseline cachexia and had higher risk baseline characteristics. Cachexia was associated with higher mortality during LVAD support (unadjusted hazard ratio [HR], 1.36 [95% CI, 1.18–1.56]; P <0.0001), persisting after adjustment for baseline characteristics (adjusted HR, 1.23 [95% CI, 1.0–1.42]; P =0.005). Mean weight change at 12 months was +3.9±9.4 kg. Across the cohort, weight gain ≥5% during the first 3 months of LVAD support was associated with lower mortality (unadjusted HR, 0.90 [95% CI, 0.84–0.98]; P =0.012; adjusted HR, 0.89 [95% CI, 0.82–0.97]; P =0.006). Conclusions The proportion of LVAD recipients recognized to have cachexia preimplantation was low at 2.5%. Recognized cachexia was independently associated with higher mortality during LVAD support. Early weight gain ≥5% was independently associated with lower mortality during subsequent LVAD support.