The aim of this study is to investigate the relationship among Working Memory Capacity (WMC) and Mathematical Performance (MP) of a sample of Pakistani students. A quantitative research approach was used to find out participants' WMC and to discover its relationship with their MP. A sample of 1186 participants' from fifty seven public sector secondary schools in Lahore, through multistage random sampling was selected for this research. Figure Intersection Test was used to measure WMC where as Mathematics Performance Test was used to measure MP. Data were analyzed by using computer software SPSS version 15. The results showed a notable difference in WMC and MP due to gender. Performance of male students on Mathematics Performance Test was better than the performance of female students in the same test. In addition, WMC and MP of English medium students was better than Urdu medium students. Overall, WMC was highly connected with MP. Three groups of WMC (low, medium and high) were significantly different from each other in scores of both Figure Intersection Test and Mathematics Performance Test. Low performance in mathematics is may be due to overloaded working memory; therefore, this study has recommended that curriculum designers may take advantage of this research while selecting mathematical topics for nine graders.