Background: Lymphopenia has been associated with in-hospital, early, and late mortality. We aimed to elucidate differences in baseline characteristics in patients with lymphopenia with and without acute respiratory distress syndrome (ARDS) and determine predictors of in-hospital mortality in this patient population. Methods: Patients ≥ 18 years of age with lymphopenia were identified in the National Inpatient Sample (2017–2021) and stratified according to ARDS diagnosis. Predictors of in-hospital mortality were determined using multivariate analyses with a logistic regression model. Results: From 183,185 patients with lymphopenia, 10,420 (5.7%) had ARDS, of which 92.8% had coronavirus disease 2019. The patients with ARDS suffered from more in-hospital mortality (47% versus 6.7%, p < 0.001). ARDS increased the odds of in-hospital mortality by eight-fold (odds ratio [OR]: 7.91 [7.06–8.86], p < 0.001). Age ≥ 65 years (OR: 4.88 [3.98–5.99]), moderate/severe liver disease (OR: 2.53 [1.87–3.42]), and metastatic cancer (OR: 2.18 [1.68–2.82]) were among the strongest positive predictors of in-hospital mortality (all p < 0.001). Conclusions: Patients with lymphopenia who have ARDS have higher in-hospital mortality, likely due to the condition’s clinical course. Lymphopenia may be a marker of immune dysregulation and systemic involvement in ARDS.