Even though linguistic or grammatical competence has been of paramount importance for language development and research in English language teaching and learning, other related competence types (i.e., pragmatic competence and sociolinguistic competence) should not be neglected. The objectives of this paper were to investigate Thai university learners' understanding of mock politeness expressions in English, comparing English major and non-English major students, and to discern the factors influencing their understanding. The data were collected from a stratified purposive sample of bachelor degree English majors and non-English majors enrolled in a public university in Bangkok. A mixed method approach was used; the quantitative data were collected from a mock politeness judgement task while the qualitative data were obtained from individual interviews. The qualitative findings revealed that the majority of both English majors and non-English majors seemed to understand English mock politeness without much difficulty, yet details of each individual's understanding differed in details. As for quantitative findings, an independent sample t-test revealed no significant differences between the accurate answers of majors and non-English majors, while a one-way ANOVA test revealed no significant differences in the accuracy of answers among the four years of study/enrollment of both majors and non-English majors. The participants' understanding of mock politeness in English is shaped by their differences in English proficiency, anxiety levels, and views regarding the importance of understanding English mock politeness, opinions about cultural influence, and strategies used for learning mock politeness. This study sheds some light for language teachers and policymakers, highlighting pragmatic or implicit meaning in English language use in general, and promoting appropriate strategies for teaching and learning different types of politeness and impoliteness in communications, including mock politeness. This enables learners to become more successful in understanding and using the English language accurately.