Background: Cancer patients treated with immune check point inhibitors are at risk of developing severe colitis. However, the efficacy and safety of treatment of severe colitis is poorly understood. Aims: To explore the safety and efficacy of infliximab and corticosteroids in severe immune-mediated enterocolitis (IMC). Method: We performed a nationwide retrospective cohort study on 140 cancer patients treated with infliximab due to IMC in Denmark from 2011 to 2021. Results: The rate of complete remission with infliximab was 52% after one dose, increasing to 73% after two or more doses. Thirteen patients (10%) required additional treatment with vedolizumab. Patients were heavily exposed to corticosteroids and received a median accumulated dose of 3978 mg (interquartile range [IQR] 2552–6414). Age- and cancer-adjusted Cox regression analysis found that a high dose of prednisolone at start of tapering ≥75 mg/day was associated with increased mortality (HR 1.67, 1.04–2.69, p = 0.035). Patients responding to infliximab experienced an improvement of symptoms after 3 days (IQR 2–4) and complete remission after 31 days (IQR 14–61). Twenty-four percent required hospitalisation for infection during treatment for IMC, lasting 7 days (median). Secondary gastrointestinal infections occurred in 16%, with Clostridioides difficile being most common (64%). Further, 10% had a thromboembolic event during the first 90 days after infliximab treatment. Conclusions: Infliximab led to complete resolution of symptoms in 73% of patients with IMC. High prednisolone dose at tapering was associated with increased mortality rate and a high incidence of infections and hospitalisations in patients with severe IMC. We suggest optimised infliximab treatment before escalation of steroid doses.