The phenomenon of code-mixing in English as a Foreign Language (EFL) learning has become an intriguing subject in sociolinguistic and language education studies. This research aims to analyze the factors influencing code-mixing in EFL classrooms and its impact on the learning process. A qualitative method with a descriptive case study design was employed, involving classroom observations and semi-structured interviews with students and teachers at SMPN 30 Semarang. The findings reveal that linguistic factors (vocabulary limitations, ease of using the mother tongue), social factors (cultural norms, societal expectations, speaking habits), and psychological factors (comfort, confidence, social identity) contribute to the use of code-mixing in English learning. The impact of code-mixing is twofold: on the one hand, it helps students understand lessons better, enhances classroom engagement, and enriches vocabulary; on the other hand, excessive dependence on code-mixing may hinder the development of independent English-speaking skills. The implications of this study highlight the importance of balancing code-mixing as a pedagogical strategy. Teachers are encouraged to use code-mixing strategically to support students' comprehension without diminishing their opportunities to practice full English communication. This study also recommends further research to explore the long-term effects of code-mixing on students' language proficiency.