Immune effector cell (IEC) therapies such as CAR-T and bispecific T-cell engagers have transformed treatment for hematologic malignancies but require intensive monitoring due to risks such as cytokine release syndrome and neurotoxicity. Psychiatric comorbidities may further influence outcomes. This retrospective study of 84 adults treated with IEC therapy from 2020–2024 examined whether preexisting mental health disorders (MHDs) and psychiatric pharmacotherapy were associated with hospital length of stay (LOS) within 30 days of therapy. MHDs were defined as documented DSM-5 diagnoses prior to treatment, and pharmacotherapy was defined as consistent use before, during, and after admission. Gamma regression models showed significantly longer LOS among CAR-T recipients compared with BiTE recipients, as well as prolonged LOS among patients with preexisting MHDs. Psychiatric pharmacotherapy was associated with shorter LOS, suggesting a potential protective effect. A grouped model demonstrated the longest LOS among patients with MHDs who were not receiving psychiatric medications. These findings highlight the importance of psychiatric assessment and optimized mental health treatment in IEC therapy recipients. Future prospective studies should evaluate whether proactive psychiatric interventions can reduce hospitalization burden and improve clinical efficiency.