Background: Sinonasal cancer is considered a rare disease with poor survival. Its treatment has changed profoundly in recent years, primarily following the introduction of intensity-modulated radiation therapy (IMRT) and minimally invasive endoscopic surgery. Danish national guidelines on treatment of patients diagnosed with sinonasal carcinoma were introduced in 2007. The aim of this phase-4 study was to assess the effect of the implementation of guidelines by describing treatment outcomes in a consecutive nationwide cohort. Methods: All patients diagnosed with sinonasal carcinoma in Denmark from 2008 to 2015 were identified in the nationwide clinical database, DAHANCA, and were followed until May 2020. Overall survival (OS) was analysed using Kaplan–Meier estimator. Cumulative incidence of locoregional failure (LRF) and disease-specific mortality (DSM) were analysed using the Aalen–Johansen estimator. Competing risks were death from other causes (DSM) and distant failure and death (LRF). Analysis of prognostic factors was performed using Cox proportional hazard analysis. Start of follow-up was time of diagnosis. The results are presented as estimates with 95% confidence intervals (95% CIs). Results: A total of 331 patients were identified. Curatively intended treatment was performed in 264 patients (80%). Non-compliance with treatment guidelines was registered in 24 patients (9%). Non-compliance was associated with LRF (hazard ratio [HR], 2.0 [95% CI: 1.1–3.5]). Among patients qualified for curative treatment, failure occurred in 109 patients (41%), primarily at the primary tumour site (81%). Anatomical tumour site and disease stage were independent prognostic factors. The 5-year OS was 56% in patients treated with curative intent, and a combined treatment strategy showed reduced LRF (HR, 0.53 [95% CI: 0.30–0.92]) in a multivariate analysis. Conclusions: Guideline compliance and a combined treatment approach reduced the incidence of LRF and thereby increased OS. Our results confirm those of international studies. Treatment of sinonasal carcinoma remains a challenge that requires multidisciplinary team coordination.