This study delved into the assessment of General Mathematics performance among Grade 11 learners in the College of Sciences, Technology and Communications, Inc., (CSTC) at Atimonan, Lucena, and Sariaya Campuses, Quezon Province, Philippines. A sample of two hundred ninety (290) learners were selected using stratified random sampling from a population of one thousand one hundred seventy-five (1,175) Grade 11 learners across various CSTC campuses. Seven (7) qualified educators provided insights and validation for Supplemental Instructional Materials (SIM). Analysis revealed persistent difficulties in mathematical modeling, function operations, and problem-solving involving rational, inverse, exponential, and logarithmic functions. The mean composite score for the least mastered competencies was 4.26, indicating a high difficulty level. The SIM's validity assessment yielded a mean score of 16.29 across various focuses, indicating strong validity. Recommendations include targeted Supplemental Instructional Materials, curriculum revision, utilization of validated SIM, and prioritizing the development of tailored SIM. Future research should focus on long-term effectiveness studies and alternative teaching methods.