Few studies have verified the different causes of project delays between the owner and contractor perspectives. This article’s goal is to find what the causes of delay are and how to mitigate this delay depending on project performance. Thus, this study investigated 82 owner-side experts and 106 contractor-side experts in Tanzanian power construction projects. In successful projects (less than 10% time delay), the owners and contractors weighted similar causes such as vandalism and permits from authorities. They suggested similar mitigation strategies such as close project supervision, capacity building training, and proper logistics management. While in unsuccessful projects (more than 10% time delay), they exhibited many different responses. In particular, contractors weighted the causes incurred by changes in scope, owner’s poor supervision, delays in approval, failure in planning and designing risk more than contractors. Owners weighted the mitigation strategies such as top management support and timely procurement more than contractors. These findings will help project managers to understand owners’ and contactors’ different concerns and develop better solutions. This study mainly contributes to improving delay management in power construction projects in developing countries.