Dynamic assessment (DA), having its theoretical base in Vygotskyan Sociocultural theory of mind, is a newly developed classroom assessment through which learners are helped to perform beyond their current ability. Drawing upon the theoretical aspect of DA, the present study explored the impact of DA on the development of speaking skills. To this end, a beginner and an advanced English language learner were recruited for the present study. To collect the data, the participants narrated a set of picture stories during which they received mediation based on their Zone of Proximal Development (ZPD). Microgenetic and thematic analysis as a data analysis framework was employed to identify any possible changes in the participants' cognition development. Moreover, the participants' private speech was considered as an index of their movement toward self-regulation. The findings revealed a significant development in the participants' cognition and their movement toward further self-regulation. Furthermore, the results of thematic analysis of unstructured interviews showed their satisfaction with DA. The findings of the study call for teachers' and materials developers' attention to learners' individualized needs which require tailored mediation to help them move forward toward further self-regulation.