Background: The aim of this study was to develop a “behavioural diagnosis” of factors influencing the uptake of the MIND diet in 40-55-year olds according to the COM-B model, in order to reduce the risk of cognitive decline in later life. Comparing a Mediterranean (Italy) and non-Mediterranean (Northern Ireland) country to inform an intervention. This study also sought to identify intervention functions and behaviour change techniques (BCTs) that are likely to be effective in changing MIND diet behaviour. Methods: This was a qualitative study that was used to elicit beliefs surrounding Capability, Opportunity, Motivation and Behaviour (COM-B) with adhering to a diet associated with a reduced risk of cognitive decline, the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND diet). This study further elaborated the COM-B components into the 14 domains of the Theoretical Domains Framework (TDF) to further understand behaviour. Twenty-five Northern Irish (NI) and Italian participants were recruited onto the study, to take part in either a focus group or an interview. Participants were both male and female aged between 40-55 years. Results: Thematic analysis revealed that the main barriers to the uptake of the MIND diet were; time, work environment (opportunity), taste preference and convenience (motivation). Culture (motivation), seasonal foods and lack of family support (opportunity) to be a barrier to the Italian sample only. The main facilitators reported were; improved health, memory, planning and organisation (motivation) and access to good quality food (opportunity). Cooking skills, knowledge (capability) and heathy work lunch (opportunity) being a facilitator to the Italian sample only. Five intervention functions and fifteen BCTs were identified for possible inclusion in intervention development. Conclusions: The “behavioural diagnosis” provides comparisons and valuable insight into the personal, social and environmental factors that participants report as barriers and facilitators to the uptake of the MIND diet in the two samples. More barriers to healthy dietary change were found than facilitators. There is a need for interventions that increase capability, opportunity and motivation to aid behaviour change. The “behavioural diagnosis” from this study will be used to design a behaviour change intervention using the subsequent steps from the Behaviour Change Wheel (BCW).