English holds a significant status as a lingua franca in Sweden, offering regular contact opportunities for learners through media consumption and entertainment, starting at a young age. Exposure to English outside the walls of the formal language classroom has been extensively studied especially in conjunction with students’ motivation, vocabulary acquisition, and oral proficiency (cf. Sundqvist & Sylven, 2016). This study examines the impact of self-reported time spent on extramural English (EE) exposure on the complexity, accuracy, and fluency of written proficiency among lower-secondary students in Sweden. The research focuses on the quantitative measures of complexity and accuracy in argumentative essays (N=20) as well as quantitative measures of fluency in argumentative essays (N=53) taken from a sample of the corpus of Swedish learners of English (SLEC). The findings show that there are positive correlations between EE and complexity, as indicated by lexical diversity and lexical sophistication. Additionally, the normalized error ratio suggests that EE is associated with fewer errors in students' writing. Fluency, measured by written words per minute, aligns with students' self-reported EE contacts, with high-frequency EE users producing longer essays. While group-level findings indicate a positive influence of EE on written proficiency, individual-level analysis reveals a non-linear and complex relationship between contact with EE and written proficiency components. The study emphasizes the importance of integrating students' EE experiences into the formal language classroom to maximize the benefits of extramural English.