Colon cancer is one of the most common cancers diagnosed in England. Despite the universal healthcare system, the most deprived groups of colon cancer patients have a poorer prognosis than the more affluent patients, particularly in the short-term following diagnosis. Comorbidity is considered to be a prognostic factor in cancer outcomes. Moreover, the presence of comorbidity tends to be associated with increased levels of socio-economic deprivation. In this thesis I investigated the role that comorbidity plays in socio-economic inequalities in ninety-day mortality after diagnosis with colon cancer. My research used population-based England national cancer registry data linked with Hospital Episode Statistics (HES), National Bowel Cancer Audit (NBOCA) and Route to Diagnosis data of approximately 100,000 patients diagnosed with colon cancer between 2009 and 2013. This research emphasised the increased burden of comorbidity and multiple comorbidity among the most deprived cancer patients. The presence of comorbidity influenced the short-term mortality of the most deprived patients following diagnosis with colon cancer, but stage of diagnosis and the receipt of surgical treatment appeared to be influential factors in socio-economic inequalities in short term mortality. Accounting for time-varying proxy measures of comorbidity severity reduced some of the differences in ninety-day mortality between the most and least deprived patients with pre-existing diabetes, COPD and cardio-vascular conditions, but disparities in mortality still remained, suggesting other factors may be contributing towards inequalities between these groups of patients. The growing prevalence of multimorbidity and the comorbidity burden among cancer patients highlights a need for healthcare systems equipped and resourced for managing multiple chronic conditions simultaneously. Further investigation into healthcare utilisation and access to optimal care may provide insights into opportunities to improve outcomes of deprived patients living with chronic diseases who go on to develop cancer. Mechanisms to be explored include the interplay between deprivation, chronic disease burden, stage at cancer diagnosis and cancer treatment options.