Argumentative writing has always been challenging for the majority of EFL learners across the globe. Many researchers have investigated English major students’ difficulties in argumentative writing. However, little did we know about the difficulties non-English major students experienced. Thus, this qualitative study aimed to discover the difficulties faced by non-English major EFL learners taking the English for Communication 2 course at Mae Fah Luang University, Thailand, in argumentative writing and its contributing factors. There were 201 students from 10 different majors from Thailand and Myanmar who were purposely selected as the participants of this study. A closed-ended questionnaire and semi-structured interview were used as the data collection instruments. The data analysis indicates that the majority of the students experienced difficulties in content (generating and developing ideas), grammar (using correct tenses and various sentence structures), organization (creating a claim, convincing evidence, a concluding sentence, etc.), vocabulary (lack of vocabulary and difficulty in using academic vocabulary), and mechanics (spelling, punctuation, and capitalization). Limited practice time, topic unfamiliarity, and pre-university writing class experience are among the contributing factors to these difficulties. This finding demands further research dealing with the recommended strategies to improve argumentative writing instruction in the future.