Construction delays are recurrent worldwide, regardless of the type of work, and can result in severe impacts, such as time and cost overruns and conflicts. This work aims to present a methodology to develop mitigation measures for the causes of Delays in Construction Projects (DCPs). Prior works have explored strategies to develop these measures but systematically failed to consider that the interrelationships among causes affect the significance of the impact of each cause. The methodology proposed combines the Interpretative Structural Modelling (ISM) and Matrix Cross Impact Matrix Multiplication (MICMAC) analyses approaches and is grounded on the opinions of construction experts collected during Focus Group Interviews (FGI). First, the critical causes of DCPs are identified. Second, the ISM model is built, representing the interrelationships between the causes and their hierarchy. Third, the MICMAC analysis is performed, revealing the strengths of the relationships among the influencing causes. Fourth, mitigation measures are developed to address the root causes identified in the previous steps, but they are designed to target and mitigate other causes, given their hierarchical relationships, driving power, and levels of dependence. Applying this methodology to the Portuguese context revealed 16 critical causes, which were then hierarchized into six levels of influence. Inadequate bidding and contract award processes and deficient communication between parties were deemed root causes, and 23 measures were put forward to mitigate DCPs. Therefore, this study contributes to the body of knowledge by designing an innovative and practitioner-wise methodology to develop mitigation measures in relation to DCPs and revealing some measures for the Portuguese context.