At present, the role of intellectual capital in organisations is significantly more important than in the past owing to the transition from an economy based on assets to another based on knowledge. The value of intangible assets increases as that of tangible assets decreases; this allows organisations, which prioritise intellectual capital, to produce competitive advantages and achieve better performance. The purpose of this paper is to ascertain if human capital, structural capital and relational capital have a positive influence on the organisational performance of medium-sized firms in the Mexican manufacturing sector. Moreover, it is analysed if competitive advantage mediates the relation between intellectual and organisational performance. Data were obtained from a survey sent to 309 CEOs of Mexican manufacturing firms. In order to test the research hypotheses, the construct validity was tried by means of face, content, convergent, nomological and discriminant validity, for which techniques such as exploratory and confirmatory factor analysis using SPSS and AMOS, respectively, were resorted to. Later on, Partial Least Squares Structural Equation Modelling (PLS-SEM) was applied using software SmartPLS. Results showed that the three dimensions of intellectual capital have a positive and significant influence on organisational performance. Furthermore, competitive advantage mediates the relation between intellectual capital and organisational performance.