The COVID‐19 pandemic had a major impact on cancer patients and services but has been difficult to quantify. We examined how the entire cancer pathway—from incidence, presentation, diagnosis, stage, treatment and survival—was affected in Northern Ireland during April–December 2020 compared to equivalent 2018–2019 periods using retrospective, observational cancer registry data from the Northern Ireland Cancer Registry (NICR). There were 6748 cancer cases in April–December 2020 and an average 7724 patients in April–December 2018–2019. Incident cases decreased by 13% (almost 1000). Significant differences were found across age cohorts and deprivation quintiles, with reductions greatest for younger people (<55 years; 19% decrease) and less deprived (22% decrease). A higher proportion had emergency admission (16%‐to‐20%) with lower proportions diagnosed pathologically (85%‐to‐83%). There was a significant stage shift, with lower proportions of early stage (29%‐to‐25%) and higher late‐stage (21%‐to‐23%). Lower proportions received surgery (41%‐to‐38%) and radiotherapy (24%‐to‐22%) with a higher proportion not receiving treatment (29%‐to‐33%). One‐year observed‐survival decreased from 73.7% to 69.8% and 1‐year net‐survival decreased from 76.1% to 72.9%, with differences driven by five tumours; Lung (40.3%‐to‐35.0%), Head‐and‐Neck (77.4%‐to‐68.4%), Oesophageal (53.5%‐to‐42.3%), Lymphoma (81.1%‐to‐75.2%) and Uterine cancer (87.4%‐to‐80.4%). Our study reveals profound adverse impact of COVID‐19 on the entire cancer patient pathway, with 13% fewer cases, greater emergency admissions and significant stage‐shift from early to more advanced‐stage disease. There was major treatment impact with lower rates of surgery and radiotherapy and higher proportions receiving no treatment. There were significant reductions in 1‐year survival. Our study will support service recovery and protect cancer services in future pandemics or disruptions.