The interaction between teachers and students is one of the aspect that helps the students in their career grooming because teachers’ personality and their behavior is responsible for his/her effectiveness in the teaching, and plays an important role in learning process in terms of achievement and success in the classroom as well as in their entire life. From various studies, it has been proven that the personality factors such as extraversion, conscientiousness, openness to experience, neuroticism and agreeableness are the key characteristics of teachers’ personality and behavior. Based on this, the purpose of this study is to investigate the effect of teachers’ personality on students’ academic achievement particularly in mathematics subject. In order to pursue the objectives of the study, researcher used sequential explanatory mixed method approach; both quantitative and qualitative paradigms. Researcher adopted the survey research design for collection of quantitative data. In this way, Big Five Inventory (BFI) is used to identify the teacher’s personality type. This inventory comprised of forty statements about big five personality traits (eight statements about each personality trait). Moreover, the quantitative data is also collected from students’ midterm examination results from the chosen schools. In contrast, qualitative data were collected through in-depth interviews with the students of selected schools. Concerning to the sample, stratified random sampling method was adopted, where, fifty seven (57) schools were randomly selected from public sector and forty five (45) schools were selected from the private sector the participants of the study comprised of 486 public and private sector elementary 6th -8thgrade school teachers in Karachi, Pakistan. Out of them, 278 schools were selected from public sector elementary schools and the rest 208 were selected from private sector elementary schools. In addition, a mathematics score of Effect of Personality Styles of Teacher on Academic Achievement of Students in Mathematics 3224 students from both chosen private and public schools was collected for the research. For the data analysis, the quantitative part is analyzed through statistical analysis in which independent sample t-test, ANOVAs, stepwise multiple regression analysis and Corelation are computed through Factor Analysis to test the hypothesis and make predictions. Besides, qualitative data are analyzed, coded and themes are generated to get the clear picture about the research problem. In result, it is obtained that that the study variables, Extraversion, Conscientiousness, Openness to experience, Neuroticism and Agreeableness, are the significant predictor of student achievement. Since, there is strong and positive correlations are found among the variables, it implies that the teacher personality has a positive and strong association with students’ achievement in Mathematics in Karachi''s public and private schools. Other factors like teachers’ qualification and years of experience in teaching mathematics showed low associated with students’ academic achievement in Mathematics.