Construction is a complex activity as it contains several inputs to achieve the desired objective. Efficient project plan practices have a significant impact on the performance of construction companies. Several companies engage in construction activity with little or no planning. This study investigated project planning practices among the construction firms in Masaka City, Uganda with a view to understanding their performances. Qualitative and quantitative techniques in a cross-sectional approach were used in this study. Data was collected from professionals working in 20 construction firms in the city making a total of 175 participants. Using questionnaires and interview guides. The data collected were analyzed using descriptive and inferential statistics with the aid of SPSS software. Three project planning practices were considered: Safety and Resource Planning (SRP), Equipment and Material Usage Planning (EMU), and Work Scheduled and Time Planning. The results showed that none of the companies fully complied with the requirement planning practices, which led to material wastage and delays in project delivery. The regression model formulated has a coefficient of determination of 77.45%, indicating that the model has the capacity to predict about 78% of the performances of construction firms. The study concluded that the poor performance of the firms is influenced by the lack of planning practices. It is therefore recommended that strict conformance and enforcement of the project management principles throughout the lifecycle of the project should be given more attention.