Developing teachers' competencies in technology integration has recently been one of the areas of attention in teacher training. This paper presents an investigation of the development of mathematics teachers' technological pedagogical content knowledge (TPACK) through collaborative lesson design activities. The study employed a pre and post-test for non-equivalent groups quasi-experiment with three groups. Participants were secondary school mathematics teachers from Dar es Salaam -Tanzania who responded to a TPACK questionnaire before and after the intervention. Group 1 participated in collaborative lesson design teams to integrate technology and implemented the designed lesson in the classroom. Group 2 participated in lesson planning and implementation and group 3 operated conventionally. Using paired sample t-test, although all groups appear to have improved their TPACK significantly, the effect size was large for group 1 and group 2 only. When the comparison between groups and across the two points of time for data collection was done using the split-plot analysis of variance, it was found that group 1 improved the most in TPACK. These finding favoured the use of collaborative lesson designs in school-based teacher design teams. The study recommends that professional development needs to be authentic by involving teachers in routine professional activities, optimizing their peer teams to support each other within the real school contexts.