Importance: Standard treatment for resectable non-small cell lung cancer (NSCLC) includes anatomic resection with adequate lymph node dissection and adjuvant chemotherapy for appropriate patients. Historically, many patients with early-stage NSCLC have not received such treatment, which may affect the interpretation of the results of adjuvant therapy trials. Objective: To ascertain patterns of guideline-concordant treatment among patients enrolled in a US-wide screening protocol for adjuvant treatment trials for resected NSCLC. Design, Setting, and Participants: This retrospective cohort study included 2833 patients with stage IB to IIIA NSCLC (per American Joint Committee on Cancer 7th edition criteria) who enrolled in the Adjuvant Lung Cancer Enrichment Marker Identification and Sequencing Trial (ALCHEMIST) screening study (Alliance for Clinical Trials in Oncology A151216) from August 18, 2014, to April 1, 2019, and who did not enroll in a therapeutic adjuvant clinical trial; patients had tumors of at least 4 cm and/or with positive lymph nodes. Statistical analysis was conducted from June 1, 2020, through October 1, 2021. Exposures: Care patterns were ascertained overall and by sociodemographic and clinical factors, including age, sex, race and ethnicity, educational level, marital status, geography, histologic characteristics, stage, genomic variant status, smoking history, and comorbidities. Main Outcomes and Measures: Five outcomes are reported: whether patients (1) had anatomic surgical resection, (2) had adequate lymph node dissection (≥1 N1 nodal station plus ≥3 N2 nodal stations), (3) received any adjuvant chemotherapy, (4) received any cisplatin-based adjuvant chemotherapy, and (5) received at least 4 cycles of adjuvant chemotherapy. Results: Of the 2833 patients (1505 women [53%]; mean [SD] age, 66.5 [9.2] years) included in this analysis, 2697 (95%) had anatomic surgical resection, 1513 (53%) had adequate lymph node dissection, 1617 (57%) received any adjuvant chemotherapy, 1237 (44%) received at least 4 cycles of adjuvant platinum-based chemotherapy, and 965 (34%) received any cisplatin-based adjuvant chemotherapy. Rates were similar across race and ethnicity. Conclusions and Relevance: This cohort study found that among participants in a screening protocol for adjuvant clinical trials for resected early-stage NSCLC, just 53% underwent adequate lymph node dissection, and 57% received adjuvant chemotherapy, despite indications for such treatment. These results may affect the interpretation of adjuvant trials. Efforts are needed to optimize the use of proven therapies for early-stage NSCLC. Trial Registration: ClinicalTrials.gov Identifier: NCT02194738.