Background: Receipt of metastatic colon cancer (mCC) treatment for Medicare beneficiaries varies across geography, potentially due to claims processors. Medicare contracting reform consolidated legacy claims processors, known as fiscal intermediaries (Part A) and carriers (Part B) into larger entities known as Medicare Administrative Contractors (MACs), responsible for both Part A and B claims. Methods: This retrospective study used Surveillance, Epidemiology and End Results (SEER) and Medicare claims data to examine the effect of Medicare contracting reform using a difference-in-differences approach. Outcomes included the receipt of guideline-recommended chemotherapy and biologic therapy, overall survival, and mean costs for Medicare beneficiaries over age 66, diagnosed with mCC between 2005 and 2009. A generalized linear model estimated receipt of therapy and mean costs. An inverse-probability of treatment weighted Cox proportional-hazards regression modeled adjusted hazard ratios for predictors of mortality. Results: Among 4,030 mCC patients, there was no statistically significant association between MAC implementation and patient receipt of guideline chemotherapy (Odds Ratio [OR]: 1.00; 95% Confidence Interval [CI] 0.65, 1.56) or biologic therapy (OR: 0.90; 95% CI: 0.57, 1.39). No impact of MAC implementation was observed on overall survival (Hazard Ratio: 1.14; 95% CI: 0.92, 1.41) or mean total costs, regardless of number of treatment lines received. Patients diagnosed in regions where the MAC was a new entity were more likely to receive biologic therapy and had higher costs as compared to patients diagnosed in regions where a legacy contractor became the MAC. Findings were robust to changing assumptions regarding timing of MAC implementation. Conclusions: Receipt of treatment and total costs varied by geographic region in both the pre-implementation and post-implementation periods. However, Medicare contracting reform did not impact mCC patients' access to guideline therapy, survival, or total costs. Future research on geographic variation should focus on later stages of contractor consolidation and disease states vulnerable to local coverage decisions.