Drawing on Vygotskian sociocultural theory of mind and social constructivism, and adopting a sequential exploratory mixed-methods approach, this study explored the impact of online dynamic assessment (DA) on EFL learners' academic writing skills through one-on-one individual and online synchronous DA sessions over Google Docs. It also investigated the short and long term impact of the DA mediations on writing performance in new and more challenging contexts by conducting near and delayed synchronous transcendence (TR) sessions. Moreover, it explored the learners' perceptions towards the impact of online synchronous DA on academic writing skills. Three EFL university students, who were preparing themselves for the IELTS examination, were selected as the participants of the study. IELTS academic writing task 1 and task 2, marked by IELTS writing band descriptors, were used to assess the learners' academic writing skills. Moreover, a semi-structured interview was conducted to assess the learners' perceptions towards the impact of online DA on academic writing skills. Tracking the learners' microgenetic development through generated mediation and reciprocity typologies, the findings indicated academic writing development in all the four areas of task achievement, coherence and cohesion, lexicon, and grammatical range and accuracy in both writing task 1 and task 2. In the TR sessions the learners showed some difficulties in transferring some parts of their developed writing skills to more challenging contexts. Thematic analysis, used to analyse the interview data, highlighted the learners' positive perceptions towards the impact of online DA on academic writing skills.