The decades following 1980 were as important to the United States as South Korea for the introduction of concepts like reading-writing connection and reading and writing outside of the school habits to improve literacy practices for students. Ironically, this also was a period when theoretical discussions were largely dominant while few empirical studies investigated the relationships and effects of reading and writing habits on writing abilities. This study aims to investigate the correlation between reading and writing habits outside of the school context, and examine how such habits influence argumentative writing ability. A total of 240 high school students participated. Both quantitative and qualitative data were collected and analyzed within this study relying on sequential explanatory mixed methods design in order to capture a complexity of literacy practices. Questionnaires were used to measure reading and writing habits, whereas writing ability was assessed through four argumentative writing tasks. For in-depth analysis, writing tasks were divided into two types: a direct question and a source-based writing task. Semi-structured interviews and classroom observations based on ethnographic approaches were also conducted for one academic year. Accordingly, differences in argumentation ability according to task type were observed. The study identified a complicated set of results. Out of school reading and writing habits are strongly correlated each other. Such reading and writing habits outside of school effects significantly impact direct question type argumentative writing task, although not for source-based writing task. This study is meaningful because it not only examines the relationship between reading and writing habits outside of the school setting and argumentative writing ability, it also lays the foundation for follow-up studies. Such research may target reading, writing, and argumentative writing abilities given this study’s support for the necessity of teacher-designed structured process writing instruction and a set of activities for learning to write more meticulous argumentative essays.