Objectives To identify the barriers and facilitators that people with severe mental illness and people with learning disabilities may encounter when accessing cancer screening and make recommendations for implementing reasonable adjustments throughout cancer screening services. Methods and analysis An 18-month sequential, mixed-methods study comprising of two phases of work and underpinned by Normalisation Process Theory, recruiting from across the North-East and North Cumbria. The first phase aims to identify the barriers and facilitators for people with severe mental illness in accessing cervical, breast and colorectal cancer screening. A systematic review of eight databases (Part 1a; PROSPERO registration number: CRD42022331781) alongside semi-structured interviews of up to 36 people with severe mental illness (Part 1b) will occur. Additional characteristics indicating populations whose perspectives may not have been accounted for in the systematic review will be targeted in the interviews. Potential participants will be identified from a range of settings across the North-East and North Cumbria, including through social media and gatekeepers within National Health Service Trusts and charities. Interviews will be analysed using framework analysis, which will be in line with the Normalisation Process Theory. The second phase of the project (part 2a) involves triangulating the results of the systematic review and interviews with existing research previously completed with people with learning disabilities accessing cancer screening. This will be to identify population specific barriers and facilitators across people with learning disabilities and people with severe mental illness to access cancer screening services. Following triangulation, part 2b will include designing and planning a future study involving stakeholders in cancer screening to explore the feasibility, practicality, and priority for implementing the recommendations to improve person centred cancer screening services (PECCS). Ethics and dissemination This study has received Teesside University ethical approval, Health Research Authority approval (IRAS: 310622) and favourable opinion (REF: 22/PR/0793). Findings will be disseminated through a range of academic and non-academic modes including infographics, blog posts and academic publications.