Background: Clinical pathways have been introduced as tools to optimize cancer care delivery, but evidence of their value in the real world is limited. This retrospective study was performed to assess treatment patterns and clinical outcomes in patients with non-small cell lung cancer (NSCLC) before and after pathway implementation at Tennessee Oncology (TO). Methods: Chart data were abstracted for patients (≥18 years) diagnosed with Stage I-IV NSCLC who initiated first-line (1L) systemic treatment at a TO clinic and had follow-up for ³6 months or until death. Patients were divided into two cohorts: pre-pathways (treatment initiation 2014–2015) and post-pathways (treatment initiation 2016–2018). Patient characteristics, treatment patterns, and outcomes were described and compared across cohorts. An exploratory study endpoint was the evaluation of outcomes based on disease stage at diagnosis. Results: Among 501 patients (251 pre-pathways and 250 post-pathways), most had advanced or metastatic NSCLC at diagnosis (Stage III: 40%; Stage IV: 42%). Chemotherapy comprised almost all 1L systemic therapy used pre-pathways (Stage I/II: 100%; Stage III: 96%; Stage IV: 83%). Post-pathways, chemotherapy remained the most common 1L therapy in patients with Stage I/II (89%) and Stage III (72%) disease, but among patients with Stage IV disease, use of chemotherapy decreased (47%) and immuno-oncology (IO) therapy alone or in combination became common (45%). Median duration of 1L therapy was longer post-pathways in patients with Stage III (2.1 months vs 1.4 months pre-pathways; P < 0.01) and Stage IV disease (3.3 months vs 2.3 months pre-pathways; P < 0.01) but did not differ among Stage I/II patients. Median progression-free survival was significantly longer post-pathways in patients with Stage IV disease (7.0 months vs 4.2 months pre-pathways; P < 0.05), but not in other disease-stage subgroups. Median overall survival increased non-significantly post-pathways for all disease stage subgroups (Stage I/II: 26 months vs 20 months pre-pathways; Stage III: 26 months vs 20 months; Stage IV: 10 months vs 9 months). For each disease stage, rates of severe adverse events were similar between cohorts. Conclusions: While outcomes for patients diagnosed with Stage III/IV NSCLC were generally improved following the implementation of clinical pathways, this change coincided with a dramatic shift in available treatment options. Improvements post-pathways were mainly observed in patients diagnosed with advanced disease. Thus, differences in outcomes between pre-pathways and post-pathways cohorts in our study are more likely attributable to other evolving practices in cancer care, particularly the availability of newer, more effective treatments such as IO therapy as part of standard practice, than implementation of the clinical pathways.