This study explored the impact of ChatGPT on L2 academic writing by focusing on the use of conjunctive adjuncts (or “conjunctions,” e.g., and, but, since, however, as a result, on the other hand) among L1-Korean college students. A mixed-methods approach was adopted to compare a treatment group interacting with ChatGPT (n = 36) with a control group interacting with peers (n = 40) during instructional activities designed to teach these structures’ usage. Quantitative results indicated no significant difference in the frequency or types of conjunctions used between the two groups, suggesting that ChatGPT’s influence on conjunction usage patterns was minimal. However, a manual analysis revealed that the treatment group demonstrated more formal and structured uses of conjunctions, aligning with academic writing norms, while the control group more frequently deviated from these norms, by, for instance, starting sentences with coordinating conjunctions and using incorrect structures such as sentence fragments. While this study had a small sample size and a narrow focus on only one aspect of grammar, its results suggested a need to more comprehensively assess AI’s potential to complement traditional language learning methods.