Cancer is now one of the defining health and development challenges of our time. The 56 nations of the Commonwealth, home to 2·7 billion people, mirror this global burden. Recorded cancer cases across member states rose by 35% in 2008-18 and the number of deaths reached almost 1·7 million in 2018. The Lancet Oncology Commission on cancer in the Commonwealth was convened to ask a deceptively simple question: how can 56 diverse nations act collectively to close the widening gap between the burden of cancer and the capacity of each country to respond? 80 researchers, clinicians, policy makers, and civil society members, organised into five working groups and ten research streams, spent 2 years assembling the evidence base for this report. Using a validated microsimulation model, we estimate that 5-year net survival across eleven common cancers in the Commonwealth stood at just 30·7% in 2020, ranging from 4·1% in low-income countries (LICs) to 59·0% in high-income countries (HICs), driven overwhelmingly by late-stage presentation, which affects 62% of patients in LICs compared with 43% in HICs. This difference in survival is compounded by chronic underinvestment. Infrastructure gaps are equally stark: only 12% of countries meet international benchmarks for CT imaging, 22% for hospital density, and 40% for radiotherapy. Governance is underdeveloped too: barely one-third of Commonwealth countries have a national cancer control plan meeting basic quality criteria. Research investment also mirrors these inequities: between 2016 and 2023, Commonwealth countries collectively invested only US$165 per cancer-related disability adjusted life year. Yet the case for investment is unambiguous. Our modelling shows that a comprehensive scaleup of treatment, imaging, and quality of care could raise 5-year net survival across the Commonwealth from 30·7% to 52·9%, averting an estimated 5·8 million deaths and adding 113 million life-years between 2025 and 2035, at an incremental cost of $36·7 billion, delivering a return of between four and twenty dollars on every dollar invested. Crucially, this investment need not depend chiefly on external aid, which is contracting sharply at present. Our modelling for 38 low-income and middle-income Commonwealth countries suggests that up to $701 billion in additional domestic fiscal space could be mobilised between 2025 and 2030, through budget reprioritisation, macroeconomic growth, targeted taxation, and prudent borrowing. On this evidence, the Commission proposes six priority actions. These commitments are ambitious, set against an unusually turbulent geopolitical backdrop of declining official development assistance, fiscal retrenchment, and contested multilateralism. Yet it is precisely this backdrop that gives the Commonwealth its distinctive advantage: a voluntary association spanning every income level and continent, bound by common legal and regulatory traditions, capable of collective action that neither purely bilateral aid nor multilateral institutions under strain can easily replicate.