In the field of second language (L2) writing, the complexity of academic writing and challenges faced by multilingual writers when learning to write for academic purposes have been well documented (Belcher & Braine, 1995; Hinkel, 2020; Tang, 2012). This dissertation investigates the application of Dynamic Assessment (DA) in diagnosing and promoting L2 English learners' academic writing development. Grounded in Vygotskian Sociocultural Theory and its concept of the Zone of Proximal Development (ZPD), DA differs from conventional forms of assessments by introducing mediation into the assessment procedures. Learner responsiveness to the assistance indicates cognitive abilities that are in the process of formation. The goal of learner-mediator collaborative cooperation is to guide learners towards increasingly independent performance as they internalize the mediation that is sensitive to their ZPD. Although several studies have explored the effectiveness of DA in enhancing L2 learners' writing abilities (e.g., Antón, 2009; Kushki, et al., 2022; Nassaji et al., 2020; Rahimi et al., 2015; Shrestha & Coffin, 2012), little research has examined the application of DA in contexts where L2 English writers are learning to write from sources to develop their academic writing abilities. The value of tying DA procedures to a DA-informed enrichment program (e.g., Feuerstein et al., 2010) to provide more sustained writing instruction targeting learner needs and emerging writing abilities also remains underexplored. This study administered two DA procedures to the participants and engaged them in reading-writing integrated tasks in the assessments. In between the two DA sessions, the participants were randomly assigned to either an enrichment program group, where the writing instruction was ZPD-attuned and informed by the diagnoses obtained from the first DA procedure, or a non-enrichment program group that receive standard, generic writing tutoring. Towards the end of the study, both groups completed a transfer assessment which was designed with the aim to ascertain the learners' ability to transfer, or reapply their learning to a more challenging, complex writing task. The study finds that DA procedures provided insight into the source of the learners' writing difficulties and their emerging abilities, which were not identified in the non-DA where the learners performed the writing tasks independently. The differential developmental levels obtained by the two groups indicated that the ZPD-sensitive enrichment program was more conductive in promoting the learners' writing abilities than the writing instruction that did not take into account their ZPD. The genetic method of analysis shed light on the multifaceted, complex, and non-linear developmental trajectories of the focal participants. The analysis revealed the processes in which they learned to construct argumentation and use source materials in English academic writing.