The purpose of this explanatory sequential mixed-methods study was to assess the impact of the STEM integration program on third through fifth grade students’ achievement scores in mathematics and science in an urban northeastern Georgia school district. In Phase 1, a quantitative research design was used to examine and compare student achievement among third, fourth, and fifth grade students. Statistical t tests were performed to determine if there were any significant differences between the students’ achievement data who were taught with the integrated STEM program (i.e., experimental group) and their counterparts who were taught with a traditional mathematics and science curriculum (i.e., comparison / control group). Phase two encompassed the employment of qualitative research measures to provide clear and logical explanations regarding the instructional practices utilized to teach mathematics and science curriculum content to third, fourth, and fifth grade students. The qualitative research design included online questionnaires and interviews administered to four focus groups that constituted instructional liaisons and teachers from STEM and non-STEM elementary schools. Descriptive statistical data collected during the qualitative component of the research provided comprehensive and graphic rationales and justifications for the outcomes and deductions established during this investigation. There was one overarching research question and six hypotheses that were tested to determine the impact of the STEM integration program on third through fifth grade students’ achievement in mathematics and science. This study concluded with the findings, implications for practice, and recommendations for further research.