Background: Melanoma clinical trials demonstrated that completion lymph node dissection is low value for most sentinel lymph node–positive patients. Contemporaneous trials of adjuvant systemic immunotherapy and BRAF/MEK targeted therapy showed improved recurrence-free survival in high-risk sentinel lymph node–positive patients. To better understand how oncologic evidence is incorporated into practice (implementation), we evaluated factors associated with discontinuation of completion lymph node dissection and adoption of systemic treatment at United States Commission on Cancer–accredited centers. Methods: In a retrospective cohort study of adults with sentinel lymph node–positive melanoma treated from 2012 to 2017 using the National Cancer Database, we evaluated use of completion lymph node dissection and adjuvant systemic treatment using mixed-effects logistic regression, reporting results as odds ratios with 95% confidence intervals. Results: Among 10,240 sentinel lymph node–positive melanoma patients, performance of completion lymph node dissection declined from 60% to 27%. Adjuvant systemic treatment increased from 29% to 43% (37% in stage IIIA patients, 46% in IIIB–C). Completion lymph node dissection was less common with lower extremity tumors (odds ratio = 0.53, 95% confidence interval = 0.44–0.64) and more common with multiple positive sentinel lymph nodes (odds ratio = 2.36, 95% confidence interval = 2.08–2.67), treatment at a high- or moderate-volume center (odds ratiohigh = 1.49, 95% confidence interval = 1.05–2.12; odds ratiomoderate = 1.32, 95% confidence interval = 1.05–1.64), and receipt of systemic therapy (odds ratio = 1.44, 95% confidence interval = 1.27–1.63). The increased likelihood of completion lymph node dissection in patients receiving adjuvant systemic treatment persisted in the most recent study years and in patients with a single positive sentinel lymph node. Conclusion: At a population level, completion lymph node dissection declined and adjuvant systemic treatment increased, reflecting evidence-responsive care. Variation in persistent use of completion lymph node dissection and in provision of adjuvant treatment for lower risk patients highlights residual gaps in both evidence and implementation.