The association between socioeconomic position (SEP) and non-communicable diseases (NCDs) is well established, but its role in driving multimorbidity remains unclear. Multimorbidity, defined as the co-occurrence of more than one chronic condition, is linked to higher mortality and reduced quality of life. This study investigates the association between SEP and multimorbidity using data from the European Prospective Investigation into Cancer and Nutrition (EPIC). Incident cases of cancer, type 2 diabetes (T2D), and cardiovascular diseases (CVDs) were analysed alongside lifestyle factors such as smoking status, alcohol intake, body mass index (BMI), physical activity and diet. Multimorbidity was defined as the presence of at least two of the studied NCDs. SEP was assessed using the Relative Index of Inequality (RII) and categorized into high, medium, and low SEP. The cohort included 277 302 participants (60.7% women) from seven countries, enrolled between 1992-2000 and followed until the first diagnosis, end of follow-up (31/12/2007), or death. For transitions to multimorbidity, follow-up extended from the first diagnosis to the second diagnosis, end of follow-up, or death. Multistate models were used to examine the nine possible transitions to first diagnoses and multimorbidity combinations. Lifestyle factors were risk factors for all the transitions, except alcohol intake. In the main model, not stratified by sex, low SEP was associated with higher risks of progressing from cancer to CVD (Hazard Ratio (HR): 1.23, CI: 1.02-1.50), CVD to T2D (HR: 1.35, CI: 1.07-1.71), and cancer to T2D (HR: 1.37, CI: 1.10-1.69). These findings highlight the persistent influence of social inequalities on the risk of multimorbidity, even in individuals with an existing chronic condition.