The current study aimed to examine the effects of topic familiarity and language proficiency on linguistic complexity, accuracy, and fluency of argumentative essays in an EFL context. This study involved 64 college freshmen, who were divided into two groups according to a TOEIC score of 700, which corresponds to the B1 level on the CEFR scale: a high group (n = 31) and an intermediate group (n = 33). Participants were asked to write two argumentative essays on different topics, controlling for order effects through counterbalancing: one familiar (driving) and the other unfamiliar (smoking). They also completed a questionnaire that included background information and a topic familiarity rating on a 10-point Likert scale. The participants’ writing samples were analyzed in terms of lexical complexity, syntactic complexity, accuracy, and fluency. The results indicated that, in terms of topic familiarity, EFL learners tended to produce texts with lower levels of lexical and syntactic complexity, as well as reduced accuracy, when writing about unfamiliar topics. With regard to language proficiency, advanced learners demonstrated a broader vocabulary range, employed longer and more complex sentence structures, and produced more accurate and extensive texts compared to their intermediate-level peers. In-depth analysis and pedagogical implications are discussed.