Background: Surgical resection offers the best chance of survival in patients with pancreatic cancer, but those with locally advanced disease (LAPC) are usually not surgical candidates. This cohort often receives either neoadjuvant chemotherapy or chemoradiation (CRT), but unintended weight loss coupled with muscle wasting (sarcopenia) can often be observed. Here, we report on the predictive value of changes in weight and muscle mass in 147 consecutive patients with LAPC treated with neoadjuvant CRT. Methods: Clinicopathologic data were obtained via a retrospective chart review. The abdominal skeletal muscle area (SMA) at the third lumbar vertebral body was determined via computer tomographic (CT) scans as a surrogate for the muscle mass and skeletal muscle index (SMI) calculated. Uni- and multi-variable statistical tests were performed to assess for impact on survival. Results: Weight loss (14.5 vs. 20.3 months; p = 0.04) and loss of muscle mass (15.1 vs. 22.2 months; p = 0.007) were associated with poor outcomes. The highest survival was observed in patients who had neither cachectic weight loss nor sarcopenia (27 months), with improved survival seen in those who ultimately received a resection (23 vs. 10 months; p < 0.001). Cox regression revealed that either continued weight loss or continued muscle wasting (SMA reduction) was predictive of poor outcomes, whereas a sarcopenic SMI was not. Conclusions: Loss of weight and lean muscle in patients with LAPC is prognostic when persistent. Therefore, both should be assessed longitudinally and considered before surgery.