Background: Surgery for anal canal cancer (ACC) and anal margin cancer (AMC) is the only curative option after failure of chemoradiotherapy (CRT). This study aimed to determine the efficacy of surgery for ACC or AMC after failed CRT. Methods: This was a single-centre, retrospective study of 161 patients initially treated with CRT. We compared the survival rates of patients successfully treated by CRT with those of patients whose CRT failed (both surgically salvaged and treated palliatively). Results: Thirty-one patients underwent surgery with curative intent, 20 received palliative treatment after failure of CRT, and 110 had effective CRT. The 5-year overall survival (OS) rate was significantly higher among patients with successful CRT than among patients who underwent surgery with curative intent (86 vs. 66%, p < 0.001). On the other hand, the 5-year OS of patients treated with curative surgery was significantly better than that of patients who underwent palliative treatment (66 vs. 13.5%, p < 0.001). The postoperative morbidity and mortality rates were 32 and 3%, respectively. Considering patients with failed CRT, curative surgery was the only factor prognostic of favourable OS in the multivariate analysis. Conclusion: Curative surgery after failure of CRT for ACC or AMC remains an effective treatment to improve survival in two-thirds of cases, resulting in high but manageable morbidity.