Purpose: Although adjuvant systemic therapy (ST) is often recommended for the treatment of patients with high-risk, early-stage non-small cell lung carcinoma (NSCLC) after surgery, there is little evidence supporting the use of ST with stereotactic body radiotherapy (SBRT). Methods: We conducted a retrospective cohort study using a multi-institutional database to identify consecutive patients with T1-3N0M0 NSCLC treated with definitive SBRT from 2006-2015. Treatment groups were defined as those who received SBRT + ST or SBRT alone. Regional-distant failure (RDF) was analyzed with Fine and Gray competing risks regression. Progression-free (PFS) and overall survival (OS) were analyzed with the Kaplan–Meier method and Cox regression. Additional comparisons were made after 2:1 nearest-neighbor propensity-score matching on clinical risk factors. Results: We identified 54 patients who received SBRT + ST. The most common ST regimen was a platinum doublet (n = 38; 70.4%). Compared with patients receiving SBRT (n = 1269), SBRT + ST patients were younger (median age: 70 v 77 years, p < 0.001), had larger tumors (>3 cm: 38.9% v 21.6%, p = 0.02) and higher T-stage (T2-3: 42.6% v 22.5%, p = 0.002). Compared with SBRT patients, SBRT + ST patients had lower 2-year RDF (3.1% v 16.9%, p = 0.02). On multivariable analysis, SBRT + ST was associated with reduced RDF (HR: 0.15, 95%CI: 0.04–0.62), with a trend toward improved PFS (HR: 0.70, 95%CI: 0.48–1.03), but not OS (HR: 0.74, 95%CI: 0.49–1.11). After propensity-score matching, the SBRT + ST cohort demonstrated improved RDF (HR: 0.17, 95%CI: 0.04–0.76) and PFS (HR: 0.59, 95%CI: 0.38–0.93). Conclusion: In this multi-institutional analysis, adjuvant ST was independently associated with reduced RDF in early-stage NSCLC patients treated with SBRT.