The present mixed-method study attempted to investigate the efficacy of using an interactionist dynamic assessment procedure to assist university EFL learners in writing argumentative essays. It also sought to discover the differential effect of dynamic assessment in face-to-face versus web-based modes of delivering mediation. The quantitative component of the study revealed the outperformance of the experimental group on the posttest. However, the two modes of delivering mediation had no differential impact on the degree of improvement of the two experimental groups in the essay writing ability. Furthermore, with regard to the transfer tasks, the experimental groups could successfully transfer their learning to the near and far transfer tasks. The qualitative analysis of the negotiations also demonstrated an improvement in the essay writing ability of the participants over the sessions. Merging the quantitative and qualitative analyses, the study found that interactionist dynamic assessment, in both modes, can have positive effects on the learners’ ability to write better argumentative essays.