The incorporation of subject-specific technologies in teaching and learning mathematics has been extensively investigated in different areas outside Uganda. The benefits are extensive, although the adoption of these technologies has been threatened by teachers needing help to apply them effectively for instruction. While this alone would aptly justify a comprehensive assessment of its applicability on the local scene, the post-COVID-19 era dynamics escalated its urgency and relevance. Thus, third-year pre-service mathematics teachers’ knowledge and aptitude to use GeoGebra in teaching and learning geometry were developed in this study. In addition, the consequence of using this software on students’ achievement in geometry was investigated using a quantitative research approach. Ninety-four third-year pre-service mathematics teachers completed a three-week intensive capacity-building workshop using GeoGebra. Their self-ratings on their technological pedagogical content knowledge (TPACK) before and after the training were measured using a self-administered questionnaire. A total of 406 students from four schools in Central Uganda participated in the quasi-experiment, with 199 and 207 students assigned to the experimental and control groups, respectively. Pre- and post-achievement tests were meted out to the students. While descriptive statistics showed a positive change in the teachers’ technological knowledge (TK), technological pedagogical knowledge (TPK), technological content knowledge (TCK), and TPACK, independent samples t-tests revealed a substantial difference in the post-test mean scores between the two groups since the experimental group students outshone those in the control group. These findings are consistent with global literature, and thus, schools in Uganda should adopt the use of GeoGebra through a continuous capacity-building approach, while considering the challenges and limitations associated with the software.